If it's an AT&T device, and if it's unlocked, you should have no issue using it as a GoPhone, so long as the SIM card you are using is compatible with the device. If the SIM card you are using gives you an error, or you're noticing something weird with your internet speed on mobile data (assuming you haven't consumed your high-speed allowance) then you'll probably need to replace the SIM card. Other than that, you should be good to go.