Just a update regarding seat selection, i asked CP if this was correct and they sent me this reply

Thank you for messaging Cathay and for being a valued Cathay Green member. Advance check-in 48 hours before departure is an exclusive benefit for Cathay members. If you have changed your frequent flyer program from Cathay membership to British Airways Club, you will no longer enjoy this benefit. Please note that any frequent flyer program modification must be completed before check-in, as changes cannot be made once you have checked in.

If you wish your miles to be credited to your British Airways Club account, we recommend removing your Cathay membership before check-in and linking your British Airways Club membership to your booking via Manage Booking or by requesting assistance from our airport staff at the check-in counter. Thank you for your understanding.