Originally Posted by MSPeconomist

True. However, if they're still available at roughly the time boarding starts, the GA should process upgrades into those seats.

When I look at a current PNR, the number of "available" D1 seats is roughly the same as the number of unclaimed seats on the seat map. DL would *never* permit upgrades into the last D1 seat two months before departure; D1 wants to sell those seats at high prices. The seats in upgrade inventory reflect seats that DL predicts will not be sold and would otherwise fly empty or be given to a nonrev.