It's very unusual to lose a confirmed upgrade... was there an equipment change on that flight to a 2 class 757 perhaps? That's the only circumstance I can envision where they might void your confirmed upgrade...and even then as a 1K you would stand a decent shot at keeping an F seat unless there were a ton of GS or paid F or C class pax ahead of you. Sorry for the dumb question.. but are you sure your upgrade cleared in advance? What seat did you have assigned previously? In all my years of flying UA I've never once lost an upgrade that had been confirmed in advance.

I flew SFO-JFK that day too..but on the 3:55pm departure in F.