I'll add my experience to this thread. I contacted a rep through AT&T's online chat a few weeks ago. Wanted to discuss plans because I was considering going with Next. I pointed out that I'd be keeping my unlimited data plan. In that conversation, I asked to confirm the date that I could upgrade. My 2 year contract ends on Oct 18th. He looked it up and confirmed that then said that I could upgrade as of Oct 1st.
Me: Seeing that I have you online, I believe I'm eligible for an iPhone upgrade on October 17th? Just want to confirm that date and if that is correct, that is the earliest that I can order a new phone? I know some are taking a few weeks to be delivered.
AT&T: Let's take a peak at when your contract ends. Do you see the profile link up there next to wireless? click that please
AT&T: Then click the user information tab
Me: Yes
AT&T: Then you will see your 2 devices next to a picture like an ID
AT&T: under that shows the contract end date for that device.
Me: 10/17/14
AT&T: I wouldn't be able to make you eligible tonight, but you will be eligible on October 1st
AT&T: Awesome so you can do it as early as October 1st
Me: Really? I didn't know that!
AT&T: Absolutely
AT&T: What else can we look into tonight?
I was surprised and happy to hear that I could upgrade on October 1st instead of waiting until the middle of the month. My phone has issues so would like to do it as soon as possible. I chatted with someone else at AT&T today. Wanted to see what time that upgrade would be good (midnight EST or later). The person I talked to said that I have to wait until the 17th. Went back and forth with it but they said it would be that date and nothing could be done even after another AT&T rep told me the 1st. Then we discussed Next. I listed to what she had to say then pointed out to the rep that it could cost me an additional $450.00 by doing that (unlimited data plan).
Well, before we closed out the conversation I asked if she was sure that AT&T wasn't going to stand behind what the other rep told me. After some lengthy research that involved a manager, I received this reply -
AT&T: Thank you so much for waiting. I was able to confirm this with my manager that we do allow early upgrades with a two year contract for any phones for as long as it falls on the same month that you'll be eligible for an upgrade.

That means that you can upgrade 10/01/2014!
So in the end, AT&T came through. I've been with them for many years and this has been the only speed bump that I've encountered. Not bad.
I guess the only problem now is finding a store with stock. I show some Apple stores that have what I want but I have to goto an AT&T store because it's an early upgrade through them.