Domestic upgrades are not based on highest fare class in the 100, 72, and 24 hour mark for EXP, PLT, and GLD. It is based on the time you submit your upgrade (earlier the better). So, theoretically, a GLD who books a $198 RT between DFW and LGA in "O" class 8 weeks prior to flight could clear and fill up F class while an EXP who bought his ticket 23hrs before flight and paid $860.00 for his ticket, may end up in the back of the bus.