self upgrader
If CX could enable on-board paid upgrades (via handheld CC machine), this "problem" could easily go away. You just say "Mr. Smith/Chen - I see you are enjoying our larger PE seats, we're happy to let you stay there for the rest of the fight for $500, otherwise you'll have to return back to your assigned seat." Sure, CX would have to get over their only-pay-the-fare-difference policy, but maybe they'd make a few bucks too.