I will occasionally run into passengers, almost always families traveling together, who will ask me to move seats so they can be seated next to each other. I think it’s rude to ask, but will generally comply as long as I am exchanging my seat for an equivalent one. Sometimes I will refuse, for example when someone wants me to move to a bulkhead seat in first class or a non-reclining exit row seat, but to keep the peace and be nice, I will genreally comply as long as I am asked nicely.
But I’ve never been asked to downgrade cabins, especially on a 12-hr flight. Check out this account on Flyertalk of a recent incident onboard a FRA-SFO United flight. All I can say is, wow…what nerve. But it sure makes for some good reading.
If something like that happened to me I would have simply told the guy, "A gentleman gives his wife the better seat," then shook my head as I scooted past him and into my seat. Or better yet, "I’m sure the person next to your wife will be happy to switch with you."
(tip of the hat to Lucky)