This question is separate from the other thread about "trade-in value." Say someone buys an iPhone 4S and is willing to accept your iPhone 4 plus a sum of cash in a trade. Say also that you are about halfway through your AT&T contract. Is this simply a matter of restoring the two phones, swapping our the sim cards and activating them? Or is it a problem for the person who is under contract already on the original iPhone 4? Would the respective contracts be unaffected?