Originally Posted by
ROKNA
Changing the FF number requires a call to reservations or in person at airport check in desk
It's also possible to change the number yourself during online check-in.
I did this myself for my last Aer Lingus flight; I flew the outbound using my AerClub number and switched to my BAEC number when checking in for the inbound.
(Still waiting to have the Avios/TPs post in my BAEC account, but that's another story for another day)