So, to migrate phones from an old CUCM 7.1 cluster to a new CUCM 10.5 cluster seems pretty easy.
You change the option 150 addresses from your old cluster to the new cluster addresses. Then, from the old CUCM select the phones and reset them. This causes them to restart, they get the new TFTP address from DHCP, find new firmware, download it, reboot, and register with the new CUCM cluster. No Problem.
What if you have to migrate back from the CUCM 10.5 to the CUCM 7.1 ?
When I attempt this, by changing the DHCP option 150 back to the old cluster and resetting the phones, the phones show that they have the old cluster TFTP address, but will not downgrade their firmware and register back to the old CUCM. They remain registered to the new CUCM. The only way I have been able to migrate one back is to manually set it back to factory defaults and then it loads the firmware from the old CUCM and registers to it.
Does anyone know of a way to do this that does not involve manually setting the phone back to factory defaults?