You can choose a F seat whenever your upgrade clears--it may do so before 24 hours within the appropriate elite window (and PMUA flyers are going to have to get used to the fact that "window" is largely meaningless now as CO does "sweeps" inside those windows; e.g. you can be inside 96 hours as a 1K and see R inventory, but not be upgraded because the sweep hasn't run yet).

If your upgrade is still waitlisted at T-24 and you check in, you will be waitlisted and may clear sometime between then and departure. You'll be able to see that waitlist and your position on it.

But you simply can't see anywhere that you are waitlisted until T-24 when you check in and show up on the upgrade list (with your position on it). Hopefully, COdbaUA will add some kind of indication that you are indeed waitlisted instead of telling us to take it "on faith" that we're indeed actually on the waitlist--I've had several PMCO experiences where that, "Yes, you are on the waitlist," translated into, "No, you're not actually on it," when I never showed up on the visible T-24 list...