Apple website directs you to AT&T website in order to do Pay to Upgrade for Next. You can do it now to get that out of the way so when pre-order time comes, you're already Next upgrade eligible.
I just got to that screen by logging in to My Account > Devices > View Upgrade Options. You don't have to actually upgrade your device right now.
Interesting, it looks like that actually works. I just chose a new phone as if I were upgrading now. It had me make that 12th NEXT payment as a separate charge. Then I just bailed out without completing the checkout for the phone. When I go check my upgrade eligibility through Apple, it says I can do a new NEXT upgrade.
Thanks for the tip!