Originally Posted by
snert
Mrs. snert and I were flying YUL-FRA a couple of years ago - she managed to UG to J while I was stuck in Y. Despite a polite request (from her), the FA refused to allow us to swap seats mid-flight. Must have been a particularly crabby FA, given what I've read here.
They are probably either crabby or playing by the strict rules. Agreed that someone could be disruptive or playing the system by switching frequently, but if you do it once and don't ask for additional services I don't see that there is any harm.