Phone gets ext from matching its MAC address with the one stored in the PBX. So you'd need to go into SYSTEM/DEVICES AND FEATURE CODES/PHONES/<ext>/IP SETTINGS/NETWORK CONFIGURATION and clear the MAC there. If you don't have a new MAC you could shortcut by just going to SYSTEM/DEVICES AND FEATURE CODES/PHONES, when you see the list of phones in right pane just right-click and choose CLEAR PIN REGISTRATION.
