From now on, if you change your MAN CDG flight, it will trigger a reprice using current fares... and I am sure you don't want that. Your only possibility is to fly MAN CDG as ticketed and then change your ticket at CDG, which is possible according to fare rules, and it will be repriced using prices available at ticket issue (the oned you want , as written in the fare rules for "voluntary changes"

BUT, it can happen that if it is the same itinerary, not adding/removing stopovers, not changing destination, it will not be repriced, it depends on the agent I think.

Upgrade to first class is possible against flying blue miles if A bucket has availability.

To build a stopover in HKG, you should upgrade from D to J bucket... and again, I suggest you to do it after your MAN CDG