Hey all, sorry if this has been said already (I can't keep up with this thread), but here's my experience.
I had a 256GB Jet Black 7 on pre-order from att.com.
I saw zero movement from my original delivery estimate (mid October), so I decided to go the apple.com reservation option. I got a 256GB regular black reserved for retail pickup yesterday. I went in to buy, but could not due to my existing NEXT agreement on an iPhone 6s.
My NEXT agreement was upgrade eligible (I made 12 payments, and when I canceled my JB pre-order from att.com my eligibility to upgrade via NEXT was almost immediately reinstated).
The problem is that Apple will not allow you to buy a new phone on NEXT upgrade if your existing NEXT agreement is not completely paid off. I read that Apple would give you a mailer to send it in to AT&T, but that wasn't the case. The only option I had to buy the phone from the Apple retail store was to buy outright or join the Apple Upgrade program.
I drove over to an AT&T corporate store to see what could be done (such as just turning my 6S in in exchange for them wiping the remaining NEXT installment balance so I could return to Apple to buy the reserved phone), but that was not an option. All AT&T would let me do is pay the existing 6s balance in full, or upgrade to a 7 through them. They didn't have the 256GB black in stock, and I had already canceled my JB pre-order, so I ended up going with the only 256GB model they had in stock or any color or type which was 7 Rose Gold.
So, be careful about using the Apple reservation system, if you're already on a NEXT plan and plan to trade the existing device to upgrade to the new one. You're basically locked into AT&T unless you want to pay the device off.