I've got an iPhone 3G on an individual plan with AT&T. My wife is on Sprint. What I'd like to do is add her to my account, convert it to a family plan, get her the 3GS using the new line discount, and then swap my 3G to her line and her 3GS to my line. I've heard it's as easy as swapping SIM cards. Is it that straightfoward and is this legit with AT&T? thanks