I see a couple of options:

1) Call back and speak to another agent, and ask for a supervisor. I doubt a regular agent would have the ability to "backdate" waitlist requests, I'm sure there are checks built in to prevent shenanigans. Not all supervisors are created equally, so a different one might have success.

2) Try WE-CARE.

3) Try PM'ing CO Insider.

And here are a couple of observations:

1) The chance that another Platinum waitlisted for a mileage upgrade on the same flight between the precise dates of April 21 and May 8 is probably rather small. The chances that this person (or worst case, persons, including companions) will clear the waitlist and you will not is even smaller.

2) Sitting on the waitlist for 16 days without clearing does not really mean much. It's not the length of time on the waitlist that matters, it's when revenue management decides to release upgrade inventory. A year ago business travel to Asia was very light, but I believe it's picked up quite a bit since then. When CO revenue management decides with reasonable certainty that it won't sell a seat for cash, it will release it for upgrade. The closer to departure, the more precise CO can be in their analysis, so that's why we often see waitlists clearing within a week or two (or a day or two) of the flight.

3) Unfortunately, your B fare does not help your priority on the waitlist, only your status and your date of request matter. However, when you transition to the Upgrade Standby List after you check in (and after the waitlist expires at T-24), then fare class does come into play. So your B fare would bode well for your position on the standby list. Even if the flight is full in FC at this point, it would probably be unusual for all 50 BF passengers to show up, so you'd still have a good chance of clearing at the gate.

4) I'm not sure if there would have been any way to avoid this situation in the first place. When you change any aspect of a ticket prior to departure, the entire ticket has to be re-priced and re-issued. I don't know if they're able to keep the unchanged segment reservations intact or if they need to re-book them as well. I think they do need to re-build the itinerary from scratch, and if so, then there's probably no way around the "reset" of your waitlist position. If that is true, they probably should have explained that to you before processing the change.