The point is that your train ticket is a transportation contract. You have a contract with DB and SBB to get you to Brussels. You want in stead to change it to a contract with SNCF and Eurostar.
It is as if you had a flexible ticket with Lufthansa and wanted in stead to fly on KLM. You will understand that this does not work.
With railways it is the same. When you travel via France you travel on different companies than when you travel via Germany. I would stick with Germany…
What kind of fare do you actually have, and where did you buy it? DB tickets do not allow changes of dates, just refunds under some circumstances. They do not use terms like “flex fare” at all…