I've been seeing different things posted. I'm not an expert, but I think this is key. DO NOT TURN ON THE iPHONE 5! Contact AT&T and get the swap completed first. I was at the store on Sunday and they did NOT have nano SIMs. Once you activate the SIM card, I don't think it can be switched to another number. You have to call from a third phone (or go to the AT&T store) and get the SIM/IMEI numbers straightened out before you activate the iPhone 5. I hope this is accurate information and that it saves some frustration/waiting this weekend.