My impression is that DL policy, or at least redcoat policy at different major domestic airports, is not to revoke upgrades that have been confirmed in advance--i.e., you have an email--but to possibly revoke gate upgrades when a mistake has been made. OTOH, a GA once told me that she didn't want to revoke an upgrade that was already listed on GIDS because the person might have seen it already. (This was when I was erroneously denied a SDC directly into FC on an FC ticket and I had refused to accept a SDC into coach so that I wasn't booked on the flight. I questioned why my SDC request was refused when the flight obviously had had some FC seats for sale.)

If it wasn't a change of aircraft to one with fewer FC seats (different 757s?), this sounds like the GA made a mistake and cleared too many upgrades too early. People can still check in up to T-30 (and make the flight, especially if elite or in TSA Pre) at MSP; I'm not sure if the same deadline applies to purchasing a ticket on line or at the airport.

Were you seeing the 18 "open" FC seats on the seat map (inaccurate!) or did GIDS say that 18 FC seats were unclaimed, which I've always interpreted to include purchased tickets in FC or upgraded in advance that have not yet checked in? Or did you see the 18 (presumably really unsold at that time and not just those who haven't yet done OLCI but are confirmed in FC, so I would assume defined differently from the unclaimed number on GIDS) when you did OLCI (much earlier in SIN?) and saw your position at that time on the upgrade list? Even with FCM, 18 is a lot of FC seats not to give out as upgrades on an elite heavy route, so I wonder whether there could have been anticipated IROPs, in which case the GA should definitely have procrastinated on processing the upgrades and standbys.

This sounds to me like some passengers in FC (connecting from a BE ticket?) were expected to miss their connection and then did not. There also could have been a lot of SDCs onto your flight, but if they miss upgrade processing, I don't think most GAs would displace others for them or even be allowed to do so. Yet this sounds like the GA's explanation to you. Maybe the GA realized that he/she processed to upgrade list too early or maybe the new passenger was an elite with a FC ticket, not just an elite wanting a free upgrade. Or maybe it was an elite who was moved to your flight due to IROPs, in which case a favor would be appropriate.