Usually gate upgrades clear into empty FC seats (I don't know much about the process for C+ "upgrades") starting at the check in cutoff time, often T-30 but this depends on the airport. For no show FC seats, the GA should process upgrades and offload the no show passengers at the deadline to board for previously checked in and/or connecting passengers, which normally is T-10 for domestic flights.

Of course if there's upgrade inventory or empty FC seats in excess of the "rule" that DL normally holds for last minute FC ticket sales, you should clear whenever. I've been cleared on the way to the airport after being rebooked in IROPs so YMMV.