If I have a iphone 4 on AT&T not currently unlocked and I am upgrading to the iPhone 5 this weekend but looking for help first. When I upgrade to the new phone since the SIM card is smaller will the existing SIM card stay in my iphone 4 or will Best Buy keep/throw the old one out? If it stays in the phone and I want to sell it to someone shipping to Vietnam will they need my old SIM card and if so, it will be completely deactivated of my information right? They just need it to jailbreak or use on AT&T over there or however it works. Thanks in advance for the help!