JHIN seems to be based in Seattle so I would assume that he redeemed mileage with AAdvantage and not with Asiamiles. If he was with Asiamiles, he would have been able to waitlist for the CX portion of the flight, which he hadn't!

For changes, you would need to contact the airline who issued your award ticket. If they said no changes, then no changes irrespective of routings or flights. If changes of flights are permitted you would most likely need to reissue your entire ticket.