Ok this is the deal, I did massive research this weekend, here on MR and also on ATT and Apple forums, and below is what (at the time of this writing) what can/cannot be done whilst on the ATT Next plan. I will first mentioned the scenario I was in, and what I personally did...hopefully it'll shed some light.
I joined the ATT next plan Feb 2, 2014 (7 months ago) and did the 12-month plan. So in order to fullfil that agreement, and I had to wait 5 more months to get the iPhone 6. So I was presented with the following options with ATT if I wanted the iPhone 6 today:
A) Pay off the remaining att next installment payments, that are due from today, until Feb 2, 2015 (5 months from now) and that total was $190 ($33.75 a month I think).
So I had to pay $190, and ALSO trade in my iPhone 5s to ATT in order to get the iPhone 6. This scenario can ONLY BE DONE AT AN ATT STORE. Apple does not accept ATT Next trade-ins. And of course the worst thing about this is, I am at the mercy of ATT and their iPhone 6 inventory (which is quite abysmal).
B) Pay off the remaining $450 retail price of my iPhone 5s, therefore I own it and I am officially done with the next plan.
In my case I went with option B. I went to an ATT store, and purchased the phone. So in ATT's system, I am currently an active customer, but eligible for a new contract OR a new next plan. I now plan on selling my iPhone 5s (unlocked now) on Amazon.
The ONLY thing you can do at an Apple store (leveraging the next plan) is if you were in my scenario:
An active ATT customer, who is eligible for an upgrade (either 2 year deal, or signing up with next).
Apple will NOT sign you up on a **new** ATT contract leveraging Next (i.e. if my tmobile contract expired yesterday, and I walk into an Apple store wanted a new account with ATT). That can only be done at an ATT store.
That was not my case, as I was an existing ATT customer (as you are). I know it's obvious, but just putting it out there in case someone else is in doubt.
That being said, I went to my local Apple retail store which had tons of iPhones in stock, told them I was an existing ATT customer eligible for an upgrade, and I want to go via the Next plan.
I was in and out in 10 minutes. All I had to do was pay the taxes of the phone (but you knew that already) which was about $50 for the iPhone 6 64gb.
So what I have now is, a new ATT 12-month next plan which expires in Sept 20, 2015 (perfect timing, right along new iPhone release cycles) and a new iPhone 6 which I want to kiss every time I use it.
Now who won? Of course ATT wins. They made some extra money off of me due to the fact I wasn't patient enough to wait 5 months. I knew this coming in, but I said screw it. I recently lost my mother to cancer last December, and the one thing I've learned during the mourning process (and still today) is that life is indeed so short, so while you're here, be happy. I know this has nothing to do with an iPhone upgrade, let alone be in the same discussion. But personally, when decisions arise (especially financial decisions) I weigh a risk/loss/happiness formula that makes sense to me, and in the end I try to make the best decision.