Your old extension probably still exsists. Go to Maintaince, Port, Station, put in the old extension number, and if it gives you an option for status, press status and look for the Logical ID number.
Now take that Logical ID number and add 7099 (or 7100 and then subtract 1). This will give you the extension number of that port in the 4-digit set up space.
Go back to System Programming, System Renumber, Single, Extension, put in that old extension number, enter, now put in that set up space extension that you came up with above, and enter.
Now you can renumber that new phone to the old extension number.
Also be aware that if you have voice mail, the coverage is on the PORT, not the Extension number, so you will need to remove the set up space extension number, and add the old number again.
(P.S., after all that, if you know the logical id's of both phones, and the connecting cords are long enough, you could simply plug and play, swapping them.)