Success story for changing:
I used ExpertFlyer to look up open available seats and found correct booking fare. It took three calls, one to Chase (booked through UR points) - no dice, second to Cathay directly, no dice. Third call I decided to experiment with a stopover. I was able to force a stop over, and it instantly made the availability wide open. All three calls made note of the "special fares" and that changes are "treated different/not possible". It took about ten minutes for the third person to find the dates and price it out, change ended up being $100 + $6 USD.
Original: JFK-HKG - A class HKG-DAD - J class.
New: JFK-HKG - A class HKG-DAD Y class
I was able to force the stopover to retain the JFK-HKG A class, however, this was at the expense of my HKG-DAD going from J to Y. I wasn't even going to follow up with the phone agent about this since he was able to make it work to retain A class on the longhaul, and that was the most important element for me. And this was the first person who made it work. I kept rephrasing it to make sure it wasn't dropping the longhaul to Y, but even looking at updated confirmation it all went through. My stop over will be I think 3 days.
I left on my outbound first leg in late Feb, and my modified date was about 350 days after to my departure, so I was able to push this into early Feb 2020 for me.
AAARRVN8 fare basis. $850ish initial price.
Amusing sidenote: The hold music on the toll free number is so annoying - the repeat in music isn't seamless so you think the agent is getting back on the line..... and then the music starts again. Every time!!
Good luck and safe flights everyone!