The way I'm doing it tomorrow, and someone please tell me if I should do otherwirse...
Activate iPhone 5 on the new line, call AT&T just tell them to swap numbers (put my number on the iPhone 5 with the nano sim already installed) and then put my current iPhone 4S on the new number with the microsim that's already installed in my 4S.
Not sure that's what you're supposed to do based on the 5 or so threads around here.