I tried prepping a preorder on Apple.com but I want to do Next Up and that's not an option. I found some fine print on there "Customers purchasing this offer through Apple cannot add the Next Up Anytime option", so I guess I'll take a gamble that AT&T site will work smoothly tomorrow. Haven't had great luck in the past and much prefer the Apple.com or Apple Store app purchasing experience.
I've had great luck with the AT&T site the last four or five upgrades.
They usually open 'early' and I'm done with two upgrades before the Apple site or app are open.
YMMV, etc.