I apologize for asking this question since I am sure it has come up, but I have not been able to locate the answer via Google or searching the forums here. So here goes. I have an iPhone 3GS with AT&T. I am eligible for an upgrade with AT&T but want to save my upgrade for the iPhone 5. However, I am getting impatient and would like to upgrade to an iPhone 4 in the interim without burning my upgrade or extending my contract (and then sell my 3GS on eBay to mitigate the cost). What I am trying to figure out is whether this is possible by buying a used iPhone 4 on eBay. Simply put, can I buy a used iPhone 4 on eBay, swap in my SIM card from my 3GS, and use it without burning my AT&T upgrade/extending my contract? If so, do I need to buy an unlocked iPhone, or can I buy one from someone who has switched to Verizon (and thus, they are still under contract with AT&T)? Let's assume scrupulous eBay sellers, etc. - I just don't understand the activation rules/SIM card issues for switching from a 3GS to a 4 and would appreciate any assistance. Thanks in advance.