With all hosts running v5.1 it should be easy to migrate the VMs. I'd go ahead, temporarily remove one of the new hosts from vCenter (Essentials only supports up to 3 hosts) and add the "old" host. You should then be able to migrate the VMs using "Migrate Host and Datastore" (using the Web Client), without even having the old host connected to the new storage. If the hosts/CPUs are compatible, you may even be able to migrate the VMs online. Only vMotion needs to be configured properly.
André