Six days later, I'm still in upgrade hell here, though there's been some progress. No one (who is willing to tell) can say what the problem is, but bizarrely, I can't make ANY purchase from the Apple Store with my AppleID (not the PRE-APPROVED iPhone 13 preorder; not even a $9 headphone jack adapter.)
Support finally gave up and got me "in line" by creating a totally new AppleID and placing my preorder. Which suggests the problem had nothing to do with actual payment (as Goldman Sachs and Citizens Bank said from the beginning) and everything to do with Apple's ordering system. The payment request was, in fact, never made, despite the Store saying over and over that "payment authorization failed" or some other pay-side error.
So I finally have a preorder for my 13 now, BUT:
- I'm literally last in line instead of first like I was at 5am Friday, which translates to six weeks later estimated delivery.
- I lost almost a full week of my life and dozens of hours on the phone with Apple Support.
- I had to bail from the iPhone Upgrade Plan (while still owing 13 months on the previous phone).
- I had to purchase outright rather than upgrading (since Citizens Bank owns the current phone).
- I now have to figure out how to move my purchase from the new temporary AppleID to my REAL one.
- I also have to figure out how to convert my purchase into an upgrade AFTER I pay out from the loan, or I'll end up with $800 in iTunes ƒüçking Gift Cards.
... What, won't Citizens Bank takes Gift Cards as loan payments? 
- I'm at the mercy of the market to retain my old phone's value until Apple can sort all this out—I'd be not surprised at all if my 12 is worth several hundred less this time next month after the flood of other people also trading up.
- I'll still will have to sort out the multiple AppleIDs issue and get Apple to unlock my current ID so that I can make Apple Store purchases in the future.
Luckily, the Apple Support members I've worked with have been eager to help, even when they've been unsuccessful. I only hope I cost Apple Corp. as much in paying those good people as I lost in work time this week. *sigh*