not about the returns
This isn't about the return, apple will let one return or exchange anything for the full 30 days. BUT, with one's carrier, AT&T in this case, they only give the "new sub", which is what a qualified upgrade looks like, with 2 yr contract extension, 14 days to reverse the contract extension, or qualified upgrade. After that, Apple will return you phone NQA, and refund what you paid NQA, but if you want to upgrade to a different device, you're not going to show up with a qualified upgrade. One can always pay full price, no qualified upgrade required at that point - but that sort of defeats the purpose.