When using the stsadm command:

stsadm.exe -o migrateuser oldlogin “domainusername” -newlogin “domainnewusername”

We received this error message:

Value cannot be null. Parameter name: userProfileApplicationProxy

If you look in the ULS logs, you’ll find that there is an Access Denied event occuring. We were able to resolve this by going into Central Administration, selecting the User Service Application, and ensuring that the administrator account being used to run the above command has Full Control rights to the Service Application. You can find this by going to the following in Central Administration:

Application Management -> Manage Service Applications -> Select, but do not follow the link for your User Profile Sync service -> The “Permissions” button in the ribbon