Originally Posted by
jsloan
There's not enough information here to be sure, but my guess is that you purchased an instant-upgrade-eligible fare, there was upgrade space available, and, for whatever reason, UA has changed their system to allow the upgrade to take place during the booking process.
The ability to upgrade during booking with PN available on a Y/B/M fare is what I remember happening, at least in the last year or so pre-covid. I always assumed this is what the "instant" part of the instant upgrade meant. Although I thought there was a intermediate prompt asking if you wanted to confirm the upgrade or not.