This arcane info will help: That F3 exists -- or even F9 -- won't make your upgrade clear this flight. What makes it clear, is United's decision to add NF availability.

Seatcounter used to show this, but now the only way to see it online is by doing a dummy booking on united.com for free, Standard award seats. As others have said, United releases these seats to the NF category (or NC if a three-class aircraft) when it does not believe that last-minute purchasers will take them. My experience is that the fewer F seats left, the less likely the flight will go to NF availability before check-in at the gate.