These issue has been commented a thousend times here: https://www.flyertalk.com/forum/luft...ght-fares.html
For a few weeks now, you cannot do anything, the seat you get during check in is the seat you keep. Even if you try to select a worse seat, they will charge you. The only options is to do it for free before boarding, or at airport check in, among the ones thay are still available, and it is free. Not even flyers like HON or SEN can select a free seat within Europe and on light fares anymore.