The seat map being full or not doesn't mean anything as most people don't / can't select seats before OLCI.

The B seat in the front row of ET will be blocked, unless it's an ex-bmi aircraft or a newer A320. Furthermore while it is now row 6, it could be moved forward or backward. You can change seats as often as you like, the problem you have is probably because only one person is being moved. Try moving all pax to different seats (row 7), confirm then immediately go in and switch back to desired seats.