Well, they are not selling F seats for cash that’s what F0 means), so currently chances for miles upgrades are dim. But especially in F, things can change. And as they don’t oversell F, you only need one cancellation to get an empty seat. But they’ll hold on to it as long as they can. Try at the Skywards desk (underneath the escalator in B concourse) at T-60 minutes when the flight is closed or otherwise on board.