anyhow if you give them revenue like upgrade money or miles, those seats should be yours, their allowance should be "F space available".
i recently bought 3 out of 6 F seats on a 777, and the remaining seats all got used during the flight (did not bother us as we had the left side of F class to us). on the right side, the purser moved his office to 1K, and other crew members used the remaining seats during the flight.
Pilots or senior staff will likely get the F seats over miles upgrades.