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.
They don’t normally roll the cabins just to accommodate upgrades to J, but for whatever reason the algorithm cleared the upgrade in advance (possible that afterward they sold additional tickets that put the cabin into an over booked situation), and there’s no reason to downgrade anyone at that point if the space exists in F.