What I've always wondered about is what happens when you cann't support the upgrade, but Y is full. We have heard i the past that a passenger can't downgrade themselves in this scenario, do they force a down grade and IDB the passenger? do they force a downgrade ad then op-up someone else and then you get that person's seat?

Or the scariest scenario: "Hmm...you do have an SWU, we'll just use that for this LAX-SFO flight."