So I had my first experience with the DL upgrade waitlist yesterday (have been mostly buying F for my DL flights). LAX-OAK didn't clear in advance, but the upgrade was consistently offered for $52.85 right up until I checked in, at which point the offer disappeared. Wasn't offered any buy-up opportunity after checking-in, nor did I see F seats magically disappear from inventory like I'm used to seeing with UA. Ultimately cleared at the gate.
So here's my question - Does DL not offer paid upgrades post check-in, or was this just an anomaly? I'm used to UA, where the consistent practice is to undercut elite upgrades by agressively hawking F seats for cheap right up until the end of boarding.