diff --git a/nomad/job_endpoint.go b/nomad/job_endpoint.go
index 8e64a7633bc4..2d6703fb9acd 100644
--- a/nomad/job_endpoint.go
+++ b/nomad/job_endpoint.go
@@ -69,30 +69,39 @@ func (j *Job) Register(args *structs.JobRegisterRequest, reply *structs.JobRegis
 		reply.Warnings = warnings.Error()
 	}
 
+	// Lookup the job
+	snap, err := j.srv.fsm.State().Snapshot()
+	if err != nil {
+		return err
+	}
+	ws := memdb.NewWatchSet()
+	existingJob, err := snap.JobByID(ws, args.Job.ID)
+	if err != nil {
+		return err
+	}
+
+	// If EnforceIndex set, check it before trying to apply
 	if args.EnforceIndex {
-		// Lookup the job
-		snap, err := j.srv.fsm.State().Snapshot()
-		if err != nil {
-			return err
-		}
-		ws := memdb.NewWatchSet()
-		job, err := snap.JobByID(ws, args.Job.ID)
-		if err != nil {
-			return err
-		}
 		jmi := args.JobModifyIndex
-		if job != nil {
+		if existingJob != nil {
 			if jmi == 0 {
 				return fmt.Errorf("%s 0: job already exists", RegisterEnforceIndexErrPrefix)
-			} else if jmi != job.JobModifyIndex {
+			} else if jmi != existingJob.JobModifyIndex {
 				return fmt.Errorf("%s %d: job exists with conflicting job modify index: %d",
-					RegisterEnforceIndexErrPrefix, jmi, job.JobModifyIndex)
+					RegisterEnforceIndexErrPrefix, jmi, existingJob.JobModifyIndex)
 			}
 		} else if jmi != 0 {
 			return fmt.Errorf("%s %d: job does not exist", RegisterEnforceIndexErrPrefix, jmi)
 		}
 	}
 
+	// Validate job transitions if its an update
+	if existingJob != nil {
+		if err := validateJobUpdate(existingJob, args.Job); err != nil {
+			return err
+		}
+	}
+
 	// Ensure that the job has permissions for the requested Vault tokens
 	policies := args.Job.VaultPolicies()
 	if len(policies) != 0 {
@@ -890,6 +899,32 @@ func validateJob(job *structs.Job) (invalid, warnings error) {
 	return validationErrors.ErrorOrNil(), warnings
 }
 
+// validateJobUpdate ensures updates to a job are valid.
+func validateJobUpdate(old, new *structs.Job) error {
+	// Type transitions are disallowed
+	if old.Type != new.Type {
+		return fmt.Errorf("cannot update job from type %q to %q", old.Type, new.Type)
+	}
+
+	// Transitioning to/from periodic is disallowed
+	if old.IsPeriodic() && !new.IsPeriodic() {
+		return fmt.Errorf("cannot update non-periodic job to being periodic")
+	}
+	if new.IsPeriodic() && !old.IsPeriodic() {
+		return fmt.Errorf("cannot update periodic job to being non-periodic")
+	}
+
+	// Transitioning to/from parameterized is disallowed
+	if old.IsParameterized() && !new.IsParameterized() {
+		return fmt.Errorf("cannot update non-parameterized job to being parameterized")
+	}
+	if new.IsParameterized() && !old.IsParameterized() {
+		return fmt.Errorf("cannot update parameterized job to being non-parameterized")
+	}
+
+	return nil
+}
+
 // Dispatch a parameterized job.
 func (j *Job) Dispatch(args *structs.JobDispatchRequest, reply *structs.JobDispatchResponse) error {
 	if done, err := j.srv.forward("Job.Dispatch", args, args, reply); done {
diff --git a/nomad/job_endpoint_test.go b/nomad/job_endpoint_test.go
index c5918e3744d9..782b8b837040 100644
--- a/nomad/job_endpoint_test.go
+++ b/nomad/job_endpoint_test.go
@@ -2316,6 +2316,38 @@ func TestJobEndpoint_ValidateJob_UpdateWarning(t *testing.T) {
 	}
 }
 
+func TestJobEndpoint_ValidateJobUpdate(t *testing.T) {
+	old := mock.Job()
+	new := mock.Job()
+
+	if err := validateJobUpdate(old, new); err != nil {
+		t.Errorf("expected update to be valid but got: %v", err)
+	}
+
+	new.Type = "batch"
+	if err := validateJobUpdate(old, new); err == nil {
+		t.Errorf("expected err when setting new job to a different type")
+	} else {
+		t.Log(err)
+	}
+
+	new = mock.Job()
+	new.Periodic = &structs.PeriodicConfig{Enabled: true}
+	if err := validateJobUpdate(old, new); err == nil {
+		t.Errorf("expected err when setting new job to periodic")
+	} else {
+		t.Log(err)
+	}
+
+	new = mock.Job()
+	new.ParameterizedJob = &structs.ParameterizedJobConfig{}
+	if err := validateJobUpdate(old, new); err == nil {
+		t.Errorf("expected err when setting new job to parameterized")
+	} else {
+		t.Log(err)
+	}
+}
+
 func TestJobEndpoint_Dispatch(t *testing.T) {
 
 	// No requirements