I have a Sprint iPhone 4S and am content with it. The SIM is already factory unlocked to accept foreign SIM cards. I was considering buying a GPP SIM in hopes to increase the phone's retail value IF I ever sell it (it could then be used on U.S.A carriers.) My question is, is GPP a factory unlock? Or will Apple disable it after every update, sort of like a jailbreak works? If this is the case I wouldn't wang to buy it... Any suggestions? Thank you!!