Then you don't need to worry about sim hacking. You just walk into the store and buy the phone and they hook it up to your existing AT&T account. You don't need to move the sim from one phone to the other, the process of buying the phone will activate the new sim in the phone and will transfer your number to it.

There is something about "Eligibility" for existing AT&T customer. If you are eligible, you get the cheap subsidized price for the phone. If you are not eligible, then you pay more for the phone. Details here.