That flight is currently sold out in all classes. I doubt CX would clear you into "I" which is discounted business class if any seats up front open up. You'll most likely have to pay full J fare.

I'm not familiar with checking reservations on CX's website, but even if you have to become an AsiaMiles member, you should have no problem switching your FF number to AA prior to boarding (e.g., at check-in).