I have an AT&T iPhone 5, different size SIM, how do I upgrade to the full price unlocked 6s from apple. They will have to transfer my phone number and I pay AT&T even more money?
I have found myself in the same place when they changed SIM card sizes. You can buy new ATT SIM cards on eBay and once inserted in the phone you call the ATT customer service and tell them you want your plan/service moved to this new SIM. Took about 10 minutes and it was over. You new 6S, if ordered from Apple will have a SIM card in it so just call ATT and have your service/plan move to it.
I have even known AT&T to give you a new SIM card at no charge.