Need help: Before iPhone5 launch!

Currently I have 1 line at AT&T on contact.
Im going to add 2 more lines on contract with 2 new iPhone 5s.
But I only want to keep 1 new iPhone, and sell the other one.

I assume new iPhone will come with new nano SIM card inside. So I'll have to unpack, get SIM out, before I sell it, right? And after I receive both phones, do I go to AT&T store to switch from iPhone5 to my old 3GS on newly added line and get micro SIM?

Thank you in advance.

 

Sorry for confusion. The iPhone 5 that I will sell is going to be swapped with 3GS. I just hoped to avoid unwrapping of a new phone to keep it look new. Other than removing SIM from new Phone, I don't need to do anything else before selling right?