I can't get my mind the idea that a simple return of eUps or miles is sufficient if this happens on any transoceanic route. Nevermind the case of a paid J downgrade!

While it may indeed be in the T&C's that eUps are not guaranteed, it certainly seems to imply there is no disincentive for AC to overbook J. Sucker someone into paying for Latitude with an instant eUp, and send them to the back if you manage to sell more paid J before cutoff.