diff --git a/api/units.go b/api/units.go
index 67b4a9537..89e414fef 100644
--- a/api/units.go
+++ b/api/units.go
@@ -76,6 +76,7 @@ func (ur *unitsResource) set(rw http.ResponseWriter, req *http.Request, item str
 	}
 
 	var su schema.Unit
+	newUnit := false
 	dec := json.NewDecoder(req.Body)
 	err := dec.Decode(&su)
 	if err != nil {
@@ -111,6 +112,21 @@ func (ur *unitsResource) set(rw http.ResponseWriter, req *http.Request, item str
 			ur.create(rw, su.Name, &su)
 		}
 		return
+	} else if eu.Name == su.Name && len(su.Options) > 0 {
+		// There is already a unit with the same name
+		// check the hashes if they do not match then we will
+		// create a new unit with the same name and later
+		// the job will be update to this new unit.
+		// if su.Options == 0 then probably we don't want to update
+		// the Unit options but only its target state.
+		a := schema.MapSchemaUnitOptionsToUnitFile(su.Options)
+		b := schema.MapSchemaUnitOptionsToUnitFile(eu.Options)
+		newUnit = !unit.MatchUnitFiles(a, b)
+	}
+
+	if newUnit {
+		ur.create(rw, su.Name, &su)
+		return
 	}
 
 	if len(su.DesiredState) == 0 {
diff --git a/fleetctl/fleetctl.go b/fleetctl/fleetctl.go
index 69513e04f..e102d616e 100644
--- a/fleetctl/fleetctl.go
+++ b/fleetctl/fleetctl.go
@@ -103,6 +103,7 @@ var (
 		Full          bool
 		NoLegend      bool
 		NoBlock       bool
+		Replace       bool
 		BlockAttempts int
 		Fields        string
 		SSHPort       int
@@ -555,7 +556,7 @@ func getUnitFileFromTemplate(uni *unit.UnitNameInfo, fileName string) (*unit.Uni
 	}
 
 	if tmpl != nil {
-		warnOnDifferentLocalUnit(fileName, tmpl)
+		isLocalUnitDifferent(fileName, tmpl, true, false)
 		uf = schema.MapSchemaUnitOptionsToUnitFile(tmpl.Options)
 		log.Debugf("Template Unit(%s) found in registry", uni.Template)
 	} else {
@@ -632,10 +633,11 @@ func findUnits(args []string) (sus []schema.Unit, err error) {
 	return filtered, nil
 }
 
-func createUnit(name string, uf *unit.UnitFile) (*schema.Unit, error) {
+func createUnit(name string, uf *unit.UnitFile, oldUnit *schema.Unit) (*schema.Unit, error) {
 	if uf == nil {
 		return nil, fmt.Errorf("nil unit provided")
 	}
+
 	u := schema.Unit{
 		Name:    name,
 		Options: schema.MapUnitFileToSchemaUnitOptions(uf),
@@ -663,6 +665,47 @@ func createUnit(name string, uf *unit.UnitFile) (*schema.Unit, error) {
 	return &u, nil
 }
 
+// checkUnitCreation checks if the unit should be created
+// It takes a unit file path as a prameter.
+// It returns 0 on success and if the unit should be created, 1 if the
+// unit should not be created; and any error acountered
+func checkUnitCreation(arg string, old *schema.Unit) (int, error) {
+	name := unitNameMangle(arg)
+
+	// First, check if there already exists a Unit by the given name in the Registry
+	unit, err := cAPI.Unit(name)
+	if err != nil {
+		return 1, fmt.Errorf("error retrieving Unit(%s) from Registry: %v", name, err)
+	}
+
+	// check if the unit is running
+	if unit == nil {
+		if sharedFlags.Replace {
+			log.Debugf("Unit(%s) was not found in Registry", name)
+		}
+		return 0, nil
+	}
+
+	// if sharedFlags.Replace is not set then we warn
+	different, err := isLocalUnitDifferent(arg, unit, !sharedFlags.Replace, false)
+
+	// if sharedFlags.Replace is set then we fail for errors
+	if sharedFlags.Replace {
+		if err != nil {
+			return 1, err
+		} else if different {
+			*old = *unit
+			return 0, nil
+		} else {
+			stdout("Found same Unit(%s) in Registry, nothing to do", unit.Name)
+		}
+	}
+
+	log.Debugf("Found same Unit(%s) in Registry, no need to recreate it", name)
+
+	return 1, nil
+}
+
 // lazyCreateUnits iterates over a set of unit names and, for each, attempts to
 // ensure that a unit by that name exists in the Registry, by checking a number
 // of conditions and acting on the first one that succeeds, in order of:
@@ -677,17 +720,14 @@ func lazyCreateUnits(args []string) error {
 	errchan := make(chan error)
 	var wg sync.WaitGroup
 	for _, arg := range args {
+		var oldUnit schema.Unit
 		arg = maybeAppendDefaultUnitType(arg)
 		name := unitNameMangle(arg)
 
-		// First, check if there already exists a Unit by the given name in the Registry
-		u, err := cAPI.Unit(name)
+		ret, err := checkUnitCreation(arg, &oldUnit)
 		if err != nil {
-			return fmt.Errorf("error retrieving Unit(%s) from Registry: %v", name, err)
-		}
-		if u != nil {
-			log.Debugf("Found Unit(%s) in Registry, no need to recreate it", name)
-			warnOnDifferentLocalUnit(arg, u)
+			return err
+		} else if ret != 0 {
 			continue
 		}
 
@@ -699,7 +739,7 @@ func lazyCreateUnits(args []string) error {
 			return err
 		}
 
-		_, err = createUnit(name, uf)
+		_, err = createUnit(name, uf, &oldUnit)
 		if err != nil {
 			return err
 		}
@@ -726,24 +766,52 @@ func lazyCreateUnits(args []string) error {
 	return nil
 }
 
-func warnOnDifferentLocalUnit(loc string, su *schema.Unit) {
-	suf := schema.MapSchemaUnitOptionsToUnitFile(su.Options)
-	if _, err := os.Stat(loc); !os.IsNotExist(err) {
-		luf, err := getUnitFromFile(loc)
-		if err == nil && luf.Hash() != suf.Hash() {
-			stderr("WARNING: Unit %s in registry differs from local unit file %s", su.Name, loc)
-			return
+// matchLocalFileAndUnit compares a file with a Unit
+// Returns true if the contents of the file matches the unit one, false
+// otherwise; and any error ocountered
+func matchLocalFileAndUnit(file string, su *schema.Unit) (bool, error) {
+	result := false
+	a := schema.MapSchemaUnitOptionsToUnitFile(su.Options)
+
+	_, err := os.Stat(file)
+	if err == nil {
+		b, err := getUnitFromFile(file)
+		if err == nil {
+			result = unit.MatchUnitFiles(a, b)
 		}
 	}
-	if uni := unit.NewUnitNameInfo(path.Base(loc)); uni != nil && uni.IsInstance() {
-		file := path.Join(path.Dir(loc), uni.Template)
-		if _, err := os.Stat(file); !os.IsNotExist(err) {
-			tmpl, err := getUnitFromFile(file)
-			if err == nil && tmpl.Hash() != suf.Hash() {
-				stderr("WARNING: Unit %s in registry differs from local template unit file %s", su.Name, uni.Template)
-			}
+
+	return result, err
+}
+
+func isLocalUnitDifferent(file string, su *schema.Unit, warnIfDifferent bool, fatal bool) (bool, error) {
+	result, err := matchLocalFileAndUnit(file, su)
+	if err == nil {
+		if result == false && warnIfDifferent {
+			stderr("WARNING: Unit %s in registry differs from local unit file %s", su.Name, file)
 		}
+		return !result, nil
+	} else if fatal {
+		return false, err
+	}
+
+	info := unit.NewUnitNameInfo(path.Base(file))
+	if info == nil {
+		return false, fmt.Errorf("error extracting information from unit name %s", file)
+	} else if !info.IsInstance() {
+		return false, fmt.Errorf("error unit name %s does not seem to be a template unit", file)
 	}
+
+	templFile := path.Join(path.Dir(file), info.Template)
+	result, err = matchLocalFileAndUnit(templFile, su)
+	if err == nil {
+		if result == false && warnIfDifferent {
+			stderr("WARNING: Unit %s in registry differs from local template unit file %s", su.Name, info.Template)
+		}
+		return !result, nil
+	}
+
+	return false, err
 }
 
 func lazyLoadUnits(args []string) ([]*schema.Unit, error) {
diff --git a/fleetctl/load.go b/fleetctl/load.go
index 0f7b0923c..1114f946f 100644
--- a/fleetctl/load.go
+++ b/fleetctl/load.go
@@ -43,6 +43,7 @@ func init() {
 	cmdLoadUnits.Flags.BoolVar(&sharedFlags.Sign, "sign", false, "DEPRECATED - this option cannot be used")
 	cmdLoadUnits.Flags.IntVar(&sharedFlags.BlockAttempts, "block-attempts", 0, "Wait until the jobs are loaded, performing up to N attempts before giving up. A value of 0 indicates no limit. Does not apply to global units.")
 	cmdLoadUnits.Flags.BoolVar(&sharedFlags.NoBlock, "no-block", false, "Do not wait until the jobs have been loaded before exiting. Always the case for global units.")
+	cmdLoadUnits.Flags.BoolVar(&sharedFlags.Replace, "replace", false, "Replace the old unit and load the new one.")
 }
 
 func runLoadUnits(args []string) (exit int) {
diff --git a/fleetctl/start.go b/fleetctl/start.go
index 2ded29539..a4236f116 100644
--- a/fleetctl/start.go
+++ b/fleetctl/start.go
@@ -51,6 +51,7 @@ func init() {
 	cmdStartUnit.Flags.BoolVar(&sharedFlags.Sign, "sign", false, "DEPRECATED - this option cannot be used")
 	cmdStartUnit.Flags.IntVar(&sharedFlags.BlockAttempts, "block-attempts", 0, "Wait until the units are launched, performing up to N attempts before giving up. A value of 0 indicates no limit. Does not apply to global units.")
 	cmdStartUnit.Flags.BoolVar(&sharedFlags.NoBlock, "no-block", false, "Do not wait until the units have launched before exiting. Always the case for global units.")
+	cmdStartUnit.Flags.BoolVar(&sharedFlags.Replace, "replace", false, "Replace the old unit and start the new one.")
 }
 
 func runStartUnit(args []string) (exit int) {
diff --git a/fleetctl/submit.go b/fleetctl/submit.go
index cbfe03f93..8a1cde01b 100644
--- a/fleetctl/submit.go
+++ b/fleetctl/submit.go
@@ -33,6 +33,7 @@ Submit a directory of units with glob matching:
 
 func init() {
 	cmdSubmitUnit.Flags.BoolVar(&sharedFlags.Sign, "sign", false, "DEPRECATED - this option cannot be used")
+	cmdSubmitUnit.Flags.BoolVar(&sharedFlags.Replace, "replace", false, "Replace the old unit with the new one.")
 }
 
 func runSubmitUnits(args []string) (exit int) {
diff --git a/functional/unit_action_test.go b/functional/unit_action_test.go
index 78a150fd7..e6019bcfb 100644
--- a/functional/unit_action_test.go
+++ b/functional/unit_action_test.go
@@ -15,12 +15,22 @@
 package functional
 
 import (
+	"fmt"
+	"io/ioutil"
+	"os"
 	"strings"
 	"testing"
 
 	"github.com/coreos/fleet/functional/platform"
 )
 
+const (
+	tmpHelloService = "/tmp/hello.service"
+	fxtHelloService = "fixtures/units/hello.service"
+	tmpFixtures = "/tmp/fixtures"
+	numUnitsReplace = 9
+)
+
 // TestUnitRunnable is the simplest test possible, deplying a single-node
 // cluster and ensuring a unit can enter an 'active' state
 func TestUnitRunnable(t *testing.T) {
@@ -167,6 +177,42 @@ func TestUnitRestart(t *testing.T) {
 
 }
 
+// TestUnitSubmitReplace() tests whether a command "fleetctl submit --replace
+// hello.service" works or not.
+func TestUnitSubmitReplace(t *testing.T) {
+	if err := replaceUnitCommon("submit"); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := replaceUnitMultiple("submit", numUnitsReplace); err != nil {
+		t.Fatal(err)
+	}
+}
+
+// TestUnitLoadReplace() tests whether a command "fleetctl load --replace
+// hello.service" works or not.
+func TestUnitLoadReplace(t *testing.T) {
+	if err := replaceUnitCommon("load"); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := replaceUnitMultiple("load", numUnitsReplace); err != nil {
+		t.Fatal(err)
+	}
+}
+
+// TestUnitStartReplace() tests whether a command "fleetctl start --replace
+// hello.service" works or not.
+func TestUnitStartReplace(t *testing.T) {
+	if err := replaceUnitCommon("start"); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := replaceUnitMultiple("start", numUnitsReplace); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestUnitSSHActions(t *testing.T) {
 	cluster, err := platform.NewNspawnCluster("smoke")
 	if err != nil {
@@ -224,3 +270,240 @@ func TestUnitSSHActions(t *testing.T) {
 		t.Errorf("Could not find expected string in journal output:\n%s", stdout)
 	}
 }
+
+// replaceUnitCommon() tests whether a command "fleetctl {submit,load,start}
+// --replace hello.service" works or not.
+func replaceUnitCommon(cmd string) error {
+	// check if cmd is one of the supported commands.
+	listCmds := []string{"submit", "load", "start"}
+	found := false
+	for _, ccmd := range listCmds {
+		if ccmd == cmd {
+			found = true
+		}
+	}
+	if !found {
+		return fmt.Errorf("invalid command %s", cmd)
+	}
+
+	cluster, err := platform.NewNspawnCluster("smoke")
+	if err != nil {
+		return fmt.Errorf("%v", err)
+	}
+	defer cluster.Destroy()
+
+	m, err := cluster.CreateMember()
+	if err != nil {
+		return fmt.Errorf("%v", err)
+	}
+	_, err = cluster.WaitForNMachines(m, 1)
+	if err != nil {
+		return fmt.Errorf("%v", err)
+	}
+
+	// run a command for a unit and assert it shows up
+	if _, _, err := cluster.Fleetctl(m, cmd, fxtHelloService); err != nil {
+		return fmt.Errorf("Unable to %s fleet unit: %v", cmd, err)
+	}
+	stdout, _, err := cluster.Fleetctl(m, "list-units", "--no-legend")
+	if err != nil {
+		return fmt.Errorf("Failed to run list-units: %v", err)
+	}
+	units := strings.Split(strings.TrimSpace(stdout), "\n")
+	if len(units) != 1 {
+		return fmt.Errorf("Did not find 1 unit in cluster: \n%s", stdout)
+	}
+
+	// replace the unit and assert it shows up
+	err = genNewFleetService(tmpHelloService, fxtHelloService, "sleep 2", "sleep 1")
+	if err != nil {
+		return fmt.Errorf("Failed to generate a temp fleet service: %v", err)
+	}
+	if _, _, err := cluster.Fleetctl(m, cmd, "--replace", tmpHelloService); err != nil {
+		return fmt.Errorf("Unable to replace fleet unit: %v", err)
+	}
+	stdout, _, err = cluster.Fleetctl(m, "list-units", "--no-legend")
+	if err != nil {
+		return fmt.Errorf("Failed to run list-units: %v", err)
+	}
+	units = strings.Split(strings.TrimSpace(stdout), "\n")
+	if len(units) != 1 {
+		return fmt.Errorf("Did not find 1 unit in cluster: \n%s", stdout)
+	}
+	os.Remove(tmpHelloService)
+
+	if err := destroyUnitRetrying(cluster, m, fxtHelloService); err != nil {
+		return fmt.Errorf("Cannot destroy unit %v", fxtHelloService)
+	}
+
+	return nil
+}
+
+// replaceUnitMultiple() tests whether a command "fleetctl {submit,load,start}
+// --replace hello.service" works or not.
+func replaceUnitMultiple(cmd string, n int) error {
+	// check if cmd is one of the supported commands.
+	listCmds := []string{"submit", "load", "start"}
+	found := false
+	for _, ccmd := range listCmds {
+		if ccmd == cmd {
+			found = true
+		}
+	}
+	if !found {
+		return fmt.Errorf("invalid command %s", cmd)
+	}
+
+	var listUnitCmd string
+	if cmd == "submit" {
+		listUnitCmd = "list-unit-files"
+	} else {
+		listUnitCmd = "list-units"
+	}
+
+	cluster, err := platform.NewNspawnCluster("smoke")
+	if err != nil {
+		return fmt.Errorf("%v", err)
+	}
+	defer cluster.Destroy()
+
+	m, err := cluster.CreateMember()
+	if err != nil {
+		return fmt.Errorf("%v", err)
+	}
+	_, err = cluster.WaitForNMachines(m, 1)
+	if err != nil {
+		return fmt.Errorf("%v", err)
+	}
+
+	if _, err := os.Stat(tmpFixtures); os.IsNotExist(err) {
+		os.Mkdir(tmpFixtures, 0755)
+	}
+
+	var stdout string
+	for i := 1; i <= n; i++ {
+		curHelloService := fmt.Sprintf("/tmp/hello%d.service", i)
+		tmpHelloFixture := fmt.Sprintf("/tmp/fixtures/hello%d.service", i)
+
+		// generate a new service derived by fixtures, and store it under /tmp
+		err = copyFile(tmpHelloFixture, fxtHelloService)
+		if err != nil {
+			return fmt.Errorf("Failed to copy a temp fleet service: %v", err)
+		}
+
+		// run a command for a unit and assert it shows up
+		if _, _, err := cluster.Fleetctl(m, cmd, tmpHelloFixture); err != nil {
+			return fmt.Errorf("Unable to %s fleet unit: %v", cmd, err)
+		}
+
+		stdout, _, err = cluster.Fleetctl(m, listUnitCmd, "--no-legend")
+		if err != nil {
+			return fmt.Errorf("Failed to run %s: %v", listUnitCmd, err)
+		}
+		units := strings.Split(strings.TrimSpace(stdout), "\n")
+		if len(units) != i {
+			return fmt.Errorf("Did not find %d units in cluster: \n%s", i, stdout)
+		}
+
+		// generate a new service derived by fixtures, and store it under /tmp
+		err = genNewFleetService(curHelloService, fxtHelloService, "sleep 2", "sleep 1")
+		if err != nil {
+			return fmt.Errorf("Failed to generate a temp fleet service: %v", err)
+		}
+	}
+
+	for i := 1; i <= n; i++ {
+		curHelloService := fmt.Sprintf("/tmp/hello%d.service", i)
+
+		// replace the unit and assert it shows up
+		if _, _, err = cluster.Fleetctl(m, cmd, "--replace", curHelloService); err != nil {
+			return fmt.Errorf("Unable to replace fleet unit: %v", err)
+		}
+		stdout, _, err = cluster.Fleetctl(m, listUnitCmd, "--no-legend")
+		if err != nil {
+			return fmt.Errorf("Failed to run %s: %v", listUnitCmd, err)
+		}
+		units := strings.Split(strings.TrimSpace(stdout), "\n")
+		if len(units) != n {
+			return fmt.Errorf("Did not find %d units in cluster: \n%s", n, stdout)
+		}
+	}
+
+	// clean up temp services under /tmp
+	for i := 1; i <= n; i++ {
+		curHelloService := fmt.Sprintf("/tmp/hello%d.service", i)
+		os.Remove(curHelloService)
+
+		if err := destroyUnitRetrying(cluster, m, fxtHelloService); err != nil {
+			return fmt.Errorf("Cannot destroy unit %v", fxtHelloService)
+		}
+	}
+	os.Remove(tmpFixtures)
+
+	return nil
+}
+
+// copyFile()
+func copyFile(newFile, oldFile string) error {
+	input, err := ioutil.ReadFile(oldFile)
+	if err != nil {
+		return err
+	}
+	err = ioutil.WriteFile(newFile, []byte(input), 0644)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+// genNewFleetService() is a helper for generating a temporary fleet service
+// that reads from oldFile, replaces oldVal with newVal, and stores the result
+// to newFile.
+func genNewFleetService(newFile, oldFile, newVal, oldVal string) error {
+	input, err := ioutil.ReadFile(oldFile)
+	if err != nil {
+		return err
+	}
+	lines := strings.Split(string(input), "\n")
+
+	for i, line := range lines {
+		if strings.Contains(line, oldVal) {
+			lines[i] = strings.Replace(line, oldVal, newVal, len(oldVal))
+		}
+	}
+	output := strings.Join(lines, "\n")
+	err = ioutil.WriteFile(newFile, []byte(output), 0644)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+// destroyUnitRetrying() destroys the unit and ensure it disappears from the
+// unit list. It could take a little time until the unit gets destroyed.
+func destroyUnitRetrying(cluster platform.Cluster, m platform.Member, serviceFile string) error {
+	maxAttempts := 3
+	found := false
+	var stdout string
+	var err error
+	for {
+		if _, _, err := cluster.Fleetctl(m, "destroy", serviceFile); err != nil {
+			return fmt.Errorf("Failed to destroy unit: %v", err)
+		}
+		stdout, _, err = cluster.Fleetctl(m, "list-units", "--no-legend")
+		if err != nil {
+			return fmt.Errorf("Failed to run list-units: %v", err)
+		}
+		if strings.TrimSpace(stdout) == "" || maxAttempts == 0 {
+			found = true
+			break
+		}
+		maxAttempts--
+	}
+
+	if !found {
+		return fmt.Errorf("Did not find 0 units in cluster: \n%s", stdout)
+	}
+
+	return nil
+}
diff --git a/registry/job.go b/registry/job.go
index 8eeda2280..d406947ea 100644
--- a/registry/job.go
+++ b/registry/job.go
@@ -335,7 +335,7 @@ func (r *EtcdRegistry) CreateUnit(u *job.Unit) (err error) {
 	}
 
 	opts := &etcd.SetOptions{
-		PrevExist: etcd.PrevNoExist,
+		PrevExist: etcd.PrevIgnore,
 	}
 	key := r.prefixed(jobPrefix, u.Name, "object")
 	_, err = r.kAPI.Set(r.ctx(), key, val, opts)
diff --git a/unit/unit.go b/unit/unit.go
index 2da58a6bb..6c54c0b91 100644
--- a/unit/unit.go
+++ b/unit/unit.go
@@ -136,6 +136,16 @@ func (u *UnitFile) Hash() Hash {
 	return Hash(sha1.Sum(u.Bytes()))
 }
 
+// MatchUnitFiles compares two unitFiles
+// Returns true if the units match, false otherwise.
+func MatchUnitFiles(a *UnitFile, b *UnitFile) bool {
+	if a.Hash() == b.Hash() {
+		return true
+	}
+
+	return false
+}
+
 // RecognizedUnitType determines whether or not the given unit name represents
 // a recognized unit type.
 func RecognizedUnitType(name string) bool {