The phone "wanting" it's old address is normal behavior for IP phones. Instead of doing a hard reset on the phone, you can "lock-out" the IP address in the DHCP scope, then tell the phone to reset. It'll sort itself out once it realizes it can't have it's old address. You can then unlock the old IP address.
Ip phones are not much different when they are DHCP clients.
If the hosts are set to have a lease of 8 days, then at 50% of their lease or 4 days, they will contact the DHCP to renew the lease.
Per MS help file:
You can delete the lease of any DHCP client in the scope. The main reason for doing so is to remove a lease that conflicts with an IP address exclusion or a client reservation that you want to add. Deleting a lease has the same effect as if the client lease expired: the next time that client computer starts, it must enter the initialization state and obtain new TCP/IP configuration information from a DHCP server. There is nothing, however, to prevent the client from obtaining a new lease for the same IP address. You must make the address unavailable before the client requests another lease.
These are the paths to get to each CCX logs through CLI. They may be helpful if you are having issues accessing RTMT or downloading logs through it.
If you want to download them you have to prefix "file get " and you can add one of the options (re...