From two AT&T reps:
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You CAN pre-order next year's iPhone and THEN trade in your current phone.
I had the same question. I decided to test this by attempting to order a 5S with an account that already has a 5S from last year on the Next plan. This could be helpful for anyone who is looking to upgrade from a 5S to a 6 and does not want to wait in line at an AT&T store.
First I tried ordering it from store.apple.com and doing a Next upgrade. It came back and said that I had one more payment left on my account to make before I could upgrade with Next, but if I go to the AT&T store, I could pay the one month and be upgrade eligible.
To expedite this, I went onto att.com and paid off my latest bill. I went back to store.apple.com and now it says that I am eligible to upgrade, but I got the following error message: "You are eligible for a Next upgrade. However, we do not support Next upgrades on the Apple Online Store. You can purchase an iPhone with your Next upgrade by visiting an AT&T store."
I then went and tried to purchase the same 5S on AT&T's website. Here it said I was eligible as long as I traded in my current device. I kept going through the check-out process and it asked me a few questions about my current device such as "Does it turn on, is it in good condition...". After saying yes to everything and confirming eligibility, it said they would ship me a box with my new phone and I had 30 days to ship back the old one. If the condition was not good enough, they have the right to charge me some fee. I then cancelled out of everything.
In conclusion, if you are looking to pre-order your iPhone 6 online and you already have a phone on AT&T Next, I recommend you do the following:
1. Make sure you've paid 12 consecutive payments against your current phone's installment plan. If not, pay your final bill or call support (or go to an AT&T store) and see if you can make early payments against your installment plan to bring you up to 12 payments. This will make you eligible for a Next upgrade.
2. On pre-order night, don't bother with Apple.com and focus on trying to pre-order from att.com. Hopefully the process for pre-ordering the iPhone 6 will be the same experience as upgrading to any other in-stock phone. They will ship out a box to bring your old phone back along with the new phone.
3. Return the old phone within 30 days using the pre-paid box.
Once again, I'm assuming that the process for pre-ordering the iPhone 6 will give the option to send out the pre-paid box just like doing any other upgrade on att.com. Either way, I got a lot farther here than on apple.com. It also doesn't look like you can do a Next upgrade at an Apple Store. I'm going to attempt to pre-order at att.com and if it doesn't work I will have to camp out at the AT&T store on launch day. I hope it doesn't come down to that.
If anyone else can add to this, please let me know. Thanks.