Thanks. Based on the buy-flow via the app,
they already know this is already a potential problem. Via one of the many people I talked to tonight, I got a Citizens One rep, who confirmed "yeah this happens ALL the time, you'll have to talk to AppleCare+ team so they can update your serial number - we cannot". Of course the AC+ team couldn't/didn't help.
View attachment 1831659
^^ Of course, after this screen (with my current/correct serial number + IMEI), I get an error that my iPhone is not currently enrolled in the IUP and I can buy a new one outright.