I'm using a iphone 4 with att $50 go phone plan, and it's working grate. But now i want to upgrade to a 4s, but my sim card is locked with that iphone 4. I can't use the same sim card to other phones, and i have just ordered an new att sim card. Is there a way to use my new sim card to replace the old locked one? or is there a way to unlock my old sim card?