Using someone else's upgrade

So my girlfriend is due for a full upgrade now through AT&T and I'm wondering if it would be possible for her to get the new iPhone with her upgrade when it comes out and give it to me and then I give her my 3gs. What would we have to do to keep our phone numbers? Would we just have to switch sim cards or would we have to go to the Apple store or AT&T store and have them do something? Or do they not even let you do something like this? I'm not very knowledgeable about how the whole cell phone stuff works.

 

Geckotek, I saw that thread but my girlfriend and I aren't on a family plan or anything, just two totally separate contracts, so I'm still wondering how they handle that situation. I assume all they care about when subsidizing a phone is that someone is signing a new contract. So, she will sign a two-year contract with a data plan and then transfer that to my 3gs and I'll just keep my current contract and transfer that to the new iPhone? And doing this won't affect my contract at all?