If they have a J seat they will of course let you give up your F seat.
Last month I was booked CAI-FRA and a lady inquired if I would mind changing seats with here, as she wanted to sit next to her husband. I was in row 3 on an A340 (first row of J).
I asked her where she was seated and she pointed to seat 2A in First class.
I asked her if she was really shure that she wanted to do this as she downgraded herself and offered me the F seat. The pursor also gave her OK so I got to sit in F !
Thank you very much !