AF passengers are travelling in a specific car of the CDG-ZYR Thalys, and tickets are not "normal" TGV Tickets.
It is in this respect different from TGV tickets given on normal TGV as a connexion (this is done by various airlines, such as CX, for the domestic connexion in France).
I doubt it will be possible to make such a change.
Anyway do not overestimate the time you can save by going through Lille.
Assuming the CDG Thalys schedule mentioned by Airfrancefan you will be in ZYR at 9:03.
From there you can arrive at Courtrai at 10:17 if you accept a connection in Gent, or at 10:44 if you wait for the direct train (I took train schedules for mid-september).
If you go through Lille you will leave CDG at 8:25, arrive in Lille-Europe at 9:15, walk (5-10 min) to Lille Flandres and take the Courtrai train leaving at 10:09 and arriving at 10:38.
So you see you can arrive faster passing by ZYR with a connection in Gent, and 6 minutes later if you wait for the direct train from ZYR.
I donīt think it justifies trying to change your ticket !