A lot of things can change in the last few hours, you could have a dozen no shows easily on some routes, and it may only be the case at T-1.5h or less. At that point some op ups will normally have taken place (including J to FLUB). I have often seen flights going out this way.
As far as I understand it, the J to FLUB moves are done manually once under airport control. I don’t know how it would work if the flight is so oversold in J and the FLUB seats are unblocked prior to check-in.
I once watched on Expertflyer as these moves were being made. The order of events was such that I could move to FLUB if I was stood at a lounge CS desk and asked them to grab an F seat for me. Has worked both times that I tried that tactic, but being in the right place at the right time while having access to EF is key.
As GGL I have had FLUB twice over and above the two instances where I tried to move myself to FLUB with some help.