From what I understand the main destination of a Schengen visa is the country that you will spend the most period of the trip in. I've my Schengen issued from Germany - multiple entries - and I already spent three days there. If I used it to spend three days in France would that violate the definition of the main destination or not?
This rule is used to determine which consulate/member state should examine a visa application. It's not directly relevant if you already have a visa. Not going to your main destination at all could create problems because it might suggest you lied to get a visa but there is no explicit requirement to stay longer in the country that issued the visa.
Additionally, multiple-entry visas work somewhat differently (they are supposed to be used for other trips than the one you described on the application). In principle, you should get one if you have reasons to visit a country repeatedly but it's not required to go to that specific country on any given trip. Because of this, changing your plans and staying a bit longer in France is even less of a concern in your case.