You can trim standard SIM cards to fit into Micro SIM slots and it works (no really, there are even tools that make it perfect if you wish). Depending on how they handle the upgrade swap you may not need this though. I've heard of people being able to pull the phone out of the box, call AT&T and they'll handle the swap so that the eligible line keeps their SIM and they activate the new SIM straight onto your line. However, my experience (and many others) have been that they end up activating the new micro SIM on the eligible line, that becomes their SIM, then they move the iPhone IMEI back to your existing SIM. Of course there are size issues in both cases in this second scenario. Point blank. Just be ready to visit an AT&T store to be assigned new SIMs...