I've searched and couldn't find a definitive answer. I'm trying to get 4 tickets from JFK-HKG together and typically CX only releases 2 in F and 2 in J. It look like closer to departure they release a lot of unsold seats on both classes (just did a search for November 2011 and you find plenty of F seats and J seats on a lot of CX flights).
Now here are my 2 questions:
1) If I book 2 in F and 2 in J and later on 2 more seats open up on either class, do you get charged if you make the change? If you downgrade the 2 F seats to 2 J seats, do you get charged but get a miles refund?

I'm going to call BA's call center but it looks like agents are not the most reliable