Originally Posted by
porphyra
This! If F is empty in this case on a 3-cabin they would op-up some J pax to F to accommodate J pax. The reason why J was likely oversold was that the F cabin excess space was floated down to allow the oversell. This happens all the time. Not sure what the reason for the OPs issue was.
Agreed it happens all the time but in my experience if my upgrade clears at T-48 the business cabin is not already full and I can get a seat assignment. Of course it can always happen that they get slammed last minute but I am guessing with the agressive buy up offers it's going to be more and more common.