I spoke to ATT today about the same issue, and although the rep seemed uncertain, I got a call back from a floor supervisor to help clarify what I was told. (This seemed unusual since I hadn't asked to be called back. The call must have been monitored and the supervisor thought it prudent.)
Anyways, I was also told there was no way to change the Sept 21st date to save $200 and be able to pre-order. I was told that I could keep my unlimited data plan and pre-order using their NEXT program, however there is no rate plan discount similar to with the Mobile Share Plans. So my monthly bills will be what I pay now plus the NEXT installment payment. I was also told that to trade the phone in next year, after 12 months, I would need to do so at an ATT Corporate store. This means I will not be able to pre-order the iPhone 6s when it gets released next year.
This seems like a trivial inconvenience, but I have gotten the latest iPhone at launch for the past 5 years and would hate not being able to do so again this year. Hopefully the policy changes before the 12th when pre-orders start.