Last week, I tried to upgrade on DL using segment upgrades and my Silver Medallion status. At the gate on the 1st flight, they issued a boarding pass. They said there was 1 F seat on the connecting flight and would waitlist me, not issue me an upgrade. About 45 min. before departure of the connection, I went to the gate and got the boarding pass. I think I saw a list of people waiting for upgrades.
Incidentally, that flight was diverted to Denver because of a fight on board.