If I understand this right, OP wants an unlocked phone to use with his existing AT&T account, correct? Then all he needs to do is go to an Apple Store and buy an AT&T phone full price, device only. That will get you an unlocked AT&T phone. Then call AT&T or go to an AT&T store, and tell them you want to switch your account to this phone. No need to buy a T-Mobile phone, or to switch in a new SIM.