Welcome to Flyertalk and the Cathay Pacific forum !

Yes you can. You would have been charged a return F class award ticket anyway so when you have F class availability between BKK-HKG and vv, you are allowed to change it. In fact if a flight is a available with F but no seats, you can waitlist till it opens up.

The only problem is that I think Asiamiles will now charge you a fee for reissuing the ticket.