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.
On the regular booking page they don't show the assigned seats and try to charge for changes.