I’m not sure why you think row 8 is booked and not blocked. BA seat map can’t differentiate between anything blocked or actually taken, both shows the same. In regards of curtain move, there is no rule. Can happen anytime, sometimes even after on line checkin open all the way to actual boarding. Usually theoretical seating takes over the flight around T-72 hours, that is your best bet to see what is the situation about row 8. If still the same I would re-check it around T-30 hours as usually few hours before online checkin open you might not able to change/select seats and you just get a message you have to wait for OLCI. Worst case you select that time. Seat alert can be useful too also people can cancel their trip so anything is possible.