EXACT same thing happened to me this past Sunday
I was on a F fare (UPDI from M fare - not the 1K insta upgrade, but I bought F ticket)
I am in a small airport, i know the gate agents, they know me, I see a LOT of the same flight attendants etc
GA was stuck, had a GS trying to get his wife on board, I volunteered to IBD and go to Delta. After door closed she rebooked me, but in coach as i was only entitled to economy per fare rules. But the voucher she gave me, made me not care much.
Well, Delta flight was delayed, missed connection - they can't touch ticket because only other connection only has FC available. 20 min later after calling 1K desk, deal is done and I fly in FC - Delta also lost half my luggage - sort of expected
Called 1K desk after I got to hotel to figure out what happened and ensure it NEVER happens again and they told me it should have never happened and I should have been booked FC to begin with, but water under the bridge now.
TL
R - I friggin hate UPDI bullcrap - quit lying to us and just tell us what we are buying, I am a big boy, I can handle it, but knowing what you have affects many other decisions, that can quickly snowball on you.