I referenced http://travel.stackexchange.com/q/13972. Herein, suppose that many seats of higher class (eg Premium Economy or Busines Class) are empty after boarding. Also, suppose that the passenger doesn't will to pay anything.
My outright guess is no; a counterargument would be that if everyone did this, then because there aren't enough such seats, then it wouldn't be fair to the peope who failed to perceive these empty seats.
Yet what would be some arguments in favour? Has this happened? Those empty seats feel wasteful.