Aubie: I think you are right. The moment the airlines compensate you on the spot for down-gradings - passengers who sneak up a class should be made to pay the difference

Oh, hang on a minute - wouldn't that mean that some people with economy tickets would board, look round and see it was too busy in Econ for their liking and then move to Business happily payming the difference. What a pleasure that would make the boarding process....

Nick