"Does anyone know what the procedure for changing account numbers from a BA EC one to an AA one for BA reservations so that it credits to AA?"

My method to ensure credit to QF is to have the QF number entered into the booking; check on checkmytrip.com that it has been correctly entered and then to add the BA number in order to use OLCI

When I log into OLCI, it then shows me as QF/Emerald

This has worked every time for me. It seems (ime) to credit to the 1st number added

Dave