The NEC SV8100 and SV9100 have the extension swap feature that we enable by default on almost every system.  You may swap extensions by the following procedure:

Analog and Digital phones may not be swapped

1.  Lift the handset of the phone you want to swap

2.  Dial 7984100

3.  Enter the extension number you want to move to that extension you are at

4.  The phone will beep a few times in the ear piece.

5.  Hang up the handset.  The phone will reboot and the extensions will be swapped.