Like a good poster on here told me yesterday, you can check your availability for upgrade by dialing *639# from your current phone. Do that and see what it says. If it says you are eligible, I'd wait until after you get one from AT&T.
That's what I did this morning: I got my iPhone from ATT store, then cancelled my online order.
When you can cancel an order at Apple is all documented at.......the Apple web site. The order goes through several stages and once it reaches a certain point, it cannot be canceled, you must accept delivery and return it.