Apple is definitely playing games. If you go to the official reservation page, it's still not live:
https://reserve.cdn-apple.com/US/en_US/reserve/iPhone/availability?channel=1
If you add "&iPP=U" to that URL, you get a working reservation page for iPhone Upgrade Program, but you can't reserve iPhone 7 Plus models:
https://reserve.cdn-apple.com/US/en_US/reserve/iPhone/availability?channel=1&iPP=U
There's another modification to the URL that will show iPhone 7 Plus, but only people who are already enrolled in the iPhone Upgrade Program are given access after the Apple Store app or website verifies that you are eligible for an upgrade. I posted above asking for someone to help me find the variable in the URL, but nobody has come through yet.