the upper deck looked full until online check in opened for us
was flying LHR to YVR early december. bummed because the upstairs looked totally booked. much to our surprise when online check-in opened (24 h before flight) there were only 5 seats taken. so we managed to get second choice which after 64a was 62a.
i would jump on the check in exactly 24 h before your flight. you might get lucky. we did.