In Web Manager, you have to do a 3-step swap.
Renumber Extension A to a placeholder extension number
Renumber Extension B to what Extension A used to be
Renumber Extension A (the Placeholder extension number) to what Extension B used to be
Example, Ext. A is 7011, Ext. B is Ext. 7012
Renumber 7011 to 8888, renumber 7012 to 7011, renumber 8888 to 7012
Go to each phone and dial *36. The first phone you do it to will drop to "No User". Dial *36 on the 2nd phone, it will come back with the new extension number, and the first phone will come back with its number as well.
In the actual Manager program, you can have the extension number exist twice, it just throws an error until you make the 2nd change.