Sys program > Exit > SysRenumber > Single > Extensions > then enter the extensions.
Say you want to switch Extensions 100 and 200, you'll need a "dummy" number in the process, so you'll have to pick one that is unused anywhere else, try this:
Renumber 100 to 768
Renumber 200 to 100
Renumber 768 to 200.
That will give you the idea of how to renumber extensions in your system, and switch things around a little.
Pepperz at newper dot net