Basically, it is typical for DL to hold back some seats for FC comp upgrades (OU bucket inventory) until the gate (usually around T-30) and it's pretty much guaranteed that there will be at least GMs (if not DMs and PMs) still ahead of you on the upgrade list at that point (status ranks ahead of cabin purchased) on a route like DTW-MCO. They won't necessarily try to sell every seat at rock bottom prices, so I wouldn't count on that either. Upgrade pricing mostly seems to correlate with FC bucket availabilty (J/C/D/I/Z) with Z being cheapest. They may release some inventory in Z close-in, or they may not. And they could only release 1 seat (which won't help you with 2 people on a reservation).