The language states (paraphrasing): "Silvers are eligible for complimentary upgrades AS EARLY as 24 hour prior to flight". Even if there are open F seats, that doesn't mean United is going to give out upgrades. They might hold the seats for last minute buyers or try to sell the upgrades. United decides in its own time when to allocate any open F seats for complimentary upgrades. Keep your expectations very low and you won't be disappointed.