If this is on AA.com, then yes you need to wait for the ticket to be fully issued before you can alter the seat reservation. Also you need your BAEC (not AA) number to be on the booking with the right status. If you had an AA number in the dim distant past and your ticket is "exchanged", there is some logic to swap out the BAEC number. If this was issued on 001 stock, which presumably it is not, then what you can sometimes do is start the Refunds part of AA.com (proceed with caution!), enter in your original ticket number, and in the detail box below it shows the "exchanged" number.