I called Apple. I talk to a very nice lady. She was very knowledgable about the upgrade program.
1) At 12:01 PT you select the phone and it will be on "hold" for you.
2) There will be a choice to get the phone using Apple upgrade program.
3) You do the appointment. Go, to the store.
4) They will run your credit (soft pull). You will need a valid credit card and proper ID.
5) If approved, you just wait for the phone. If declined, they will give you the option to pay for the phone full price.
If declined, also to possibly select one of the carrier's programs? I don't see why they wouldn't. Main thing is that you don't loose out on the phone you reserved, but still have the carrier's options.