Another rule to consider that I've been hit by a few times (and is one I find rather user unfriendly and unnecessary) is that once you are ticketed, you cannot change your routing, which would include carriers. You can change dates and times (and downgrade or upgrade to the highest class allowed on your award -- i.e. if CX offers an F flight on the SIN-HKG route, I can upgrade to it), but you cannot change the route without canceling the whole thing and starting over, thus loosing your award space on your already exisiting flights and having to rebank the miles.

Why AA can't do this (and charge for it, as they do on All Airline awards) is beyond me.