Something similar happened to me in F recently (change in cabin size and the seat assignment moved to a non-window seat) and I couldn't fix at online check-in. Once I got to the airport, there were more seats available, so I was able to change seats at physical check-in. Ended up in 1A which was better than what I'd booked originally.

Remember a lot of people change their seats when they check-in online and/or don't even show up for the flight. Bizarre, I know, but I guess once I booked an F seat for my biz partner and he decided not to fly, so it happens.