It might not be early upgrade price. Last time AT&T provided a special iPhone upgrade price if you're one of the people with the old iPhones. However, the price required a new contract for 2 years (the old contract will be void).
Given that AT&T didn't subside the original iPhone, we could understand that. But now with subsidy, we don't know.
So, technically this doesn't answer your question, but if it goes like the 1st to 2nd gen pricing, then it will be 199/299. if it doesn't have the special 1st to 2nd then it'll be 399/499