It is really easy to migrate a phone from one model to another. Examples: 7960 to 7965 or 7940 to 7945. This is helpful if you are only swapping out a few phones at a time. The BAT tool can be used to do jobs that require many phones.
To migrate a phone to a different model, go to the device in Call Manager, and select the phone that needs to be migrated. On the phone configuration screen, in the drop down box (related links at the top right of the screen), select Migrate Phone and click go.
Just enter the MAC address, phone template, and description of the new phone and click save.
I hope this information is helpful.