I had something similar happen, after being seated in F with an e500 upgrade, the GA came on and told me that "the system" made a mistake by upgrading me, and that she needed my F seat for another pax. I told her that was no problem, as long as I got my original exit row aisle back. She told me to sit tight, left the plane and never came back. I guess she didn't need that seat afterall. My advice would be to not be so willing to give up the seat you're sitting in, and make it a little difficult for the GA to play musical chairs.