A Schengen visa has a validity period and a duration. Usually validity is longer than duration. For example, if you have 10 days duration between January 1st and January 20th, you have the choice to stay from January 1st to January 10th, from January 2nd to January 11th, ..., from January 11th to January 20th, or even from January 19th to January 20th (you do not have to use all your duration, but you cannot stay past the end of validity).
This is done so that you don't need a new visa application if you make minor changes to your itinerary, e.g. a business meeting that gets postponed by a week.
So you have to check what validity and duration you have. Usually for first-time applications you get not much more validity than duration, perhaps a day on either end.
That could become a problem in your case because you cannot hold two different Schengen visa valid at the same time. So if the Swiss gave you extra validity beyond the duration, you cannot get a new French visa for those days. You'd have to cancel the Swiss visa and apply for a new one for the entire trip.
I'd advise against that.