You are concerned and rightfully so, though SNCF will not leave you high and dry, However, it is longer and more complicated to get to Chartres. There is bus service but from the Gare Austerlitz. There is a TGV/TER service but you have to go all the way to Le Mans and change. All options are over 2.5 hours, and they are limited, so you are right to be on this early.
However, a call to RS headquarters might solve a lot of this.
I'd do that sooner rather than later since a lot of people will be facing the same challenges. They may come up with solutions for everybody.
Someone wrote somewhere about electrical work at Gare Montparnasse and Gare du Nord that is closing a lot of the tracks.