I'm confused. Why would you ever talk to the generic AsiaMiles desk when you are an MPC member? In my experience MPC can reissue to a higher class up until the flight is released to the airport (i.e. up to about 3 or 4 hours before the flight). I have had ticketing deadlines for reward bookings set at 12 hours before the flight on occasion.

On the fly you ask at check-in, and they will either do it or put you on a waitlist for upgrade. Let them know you are on the waitlist when you get to the lounge and they will page you if it clears.

(That's how it works at the ports I've tried anyway, but that doesn't include MEL)