The answer is technically no. If the ISM follows the rules to a T, flights going to/from North American airspace are not supposed to have seats swapped once the doors are closed.

In more common sense terms, assuming that rule above is willing to be broken, what midlevels says above seems to make a lot of sense to me, unfortunately for your situation. I can't see how CX would want to allow this practice at all - it could be a great way to save some serious $ and get a great sleep. Buy one ticket JFK-HKG in F and another in discount Y. I know that's not what you're doing, but it's hard to see why they would allow it. Personally I have never seen this be successful either, unfortunately - I'd be curious if any of the other folks on here who regularly fly F have. Anecdotally I tried to once swap a my J seat for a friend's Y seat in-flight midflight (long-haul, LAX-HKG), which was denied. I was a regular paid J customer (not an op-up).

Lastly, I have both seen with my own eyes and had another friend recount to me what fakecd and marcuslai say about the end of the flight. I have seen parents in F and the kids came up on descent (empty seats) for landing and deplaning on a HKG-SFO flight a few years back. One of my friends also recently recounted to me that he did something identical to that, the family was split between F/J and upon descent the ISM let them all sit together in F.