I've had this issue when trying to change seats before. Ticketed bookings, seats already booked at T-7 as Bronze and then better ones come up on the seat map.

Its often an issue, I accept it and put it down to poor IT... I once tried online chat, they assured me they'd changed it and never did. I've given up ever since.

If you're not ticketed, it won't let you book in the first place and you don't get as far as the seat map anyway.

I will try AY website next time.