Actually, duplicating you DID numbers for all the extensions is a mistake I see in the field all the time. You can allocate say 1-100 for all you fixed DID numbers that never change and 101-200 and 201-300 for the ones that do, usually only the main number. So in the mode you simply allocate 1-100 and 101-200 the in mode 2 1-100 and 201-300. That way you aren't complicating the issue for numbers that are static and the ones that aren't.