It all really depends on how far you get the migration, once youv'e upgraded the virtual hardware and or tools of the vm there is no turning back. However dont forget that the vm's are actually registered on the esxi servers. All you need to do is once youv'e built the vcenter5.1 server login to it with the vi client and add the hosts in again and I will pull in all the settings of the registered vm's with each host. Once the old vcenter has been shutdown.
If you do it that way install update manager and you can upgrade your esxi servers to 5.1 automatically.