Officially what you are trying to do is not allowed, so you may encounter resistance from agents to change your number. You can try asking them to just remove your number which would open it back up for you to add your AA FF#, but the best way is to just pick the option that you are booking for someone else when making the award reservation on BA’s website. For next time.

Also, to the subject of this thread/OP, I’ve always been able to access the reservation using the BA record locator or ticket number.