ETKT receipt is clear: CX long haul are L class, MU short-haul are M classes. I never buy tickets and classes that I can't earn miles. Double-checked, and it is M class on MU routes.

I called CX Reservation office in China for the handling. It sounds more and more like mishandling because I remember the conversation vividly. At first the agent over the phone says "you can't change" but when I say I just called earlier and asked for the price and another agent told me $100+USD penalty then she told me to "hold on", gone for 5+ minutes, and return with "Ok, I change for you"

Sounds like an error in rebooking if it's not customary to lower a class on change dates.