Original itinerary Fr 13nights
would it be okay to change itinerary to:
Change to FR 6nights-> IT 7nights -> FR 1night
Already hold Schengen visa valid for 15 days.
Will I be in breach?
That's quite a big change and you would spend as much time in Italy than in France so it's really borderline but you are still using your visa for its original purpose and France is still the main destination and the country from which you should have applied for a visa so it should be OK. For a full discussion of the rules, see Should my first trip be to the country which issued my Schengen Visa? or Can I change my route and hotel reservation after getting a Schengen visa? and all the linked questions.
That said, entering and leaving through France means it's very unlikely you will have any problem in practice. Unless they ask you about it, border guards might not even notice. Do have all the relevant documentation (hotels, list of sights you want to see, etc.) for Italy ready as well. That way, if there is any concern you can show that you really are a tourist and are not trying to abuse the visa.