Phone extensions are tied to the phone / MAC address.  You can physically move a phone to a new location and it will keep the extension assigned to it.

If you need to assign a new extension to a phone that has a different extension assigned to it, one of the easiest ways is to find the current extension in the phone system and change the MAC address.  Change the last number to something random and save.  It should immediately prompt the phone for an extension, and then you can change it to the different extension.