I don't know about the rules, but I had a similar situation when flying with my girlfriend. I was booked F because it was the last ticket on that flight and I had a $300 ecert to burn (good deal for me, really), but she was stuck back in E- (E+ was full). I just asked the purser in F if it was ok at the start of the flight, and he let us do it.

His main concern about the seat swap was that it was one time and that we wouldn't be playing musical chairs or that I would come up to F during the flight.