Was this a Market Fare ticket? If not, I would try the regular AC Manage Booking page. What I found from my experience when I tried to use ACO to manage a Fixed Mileage reward, many "strange" thing will happen. The only reason I tried the ACO web page last year is because it was a Market Fare ticket (my first and the only one so far) and the regular page want to charge me. If everything else fail, I think you will need to call in to select the seat that you want.