Going by the components, that a Jeunet Classique. Based on the decals, it's mid-late 1970s. It's equivalent to the Peugeot UO8 or a Gitane Gran Sport with the QR option. Basically, it's one step up from entry level for the era.
Jeunet was typical of French bicycles of the era. The emphasis was on a nice, resilient ride quality and realitively light weight. As a trade-off, the bicyles lacked some of the some of the durability and clean finishing of the non-French competition. This is not to say that the bicycles were not well made. It's just that the French believed that light weight was more important than a frame coming out outscathed from a collison with a dump truck and that money was better spent elsewhere than wasting time on a very nice paint job and lug finishing.
The 'big four' of French manufacturing during the era were Motobecane, Peugeot, Girtane and Mercier. Jeunet fell in to second tier, below this group. As Nick says, there were some very nice Jeunets. I had 1974 Jeunet Professionel with Reynolds 531 DB, Huret Jubilee derailleurs, TA Pro cranks, MAFAC 2000 brakes and MAVIC gold (260g) tubular rims. It was extremely light for it's day, even lighter than a Peugeot PX10LE. Unfortunartely, it did not survive a crash and I trashed the frame (big mistake) but still have the components.
Your Jeunet does not have much value, but it's appears to be in nice condition and should be a nice ride. As for changing out components, that is fine, provided you decide to keep it. However, it will decrease the value should you decide to sell it and you come across someone who used to own one 30 years ago and just has to have it. The other concern is that most French bicycles of this era used French standard threads (headsets, bottom brackets, pedals, freewhweels) and smaller diameter handlebars and stems. Replacements for these parts will be more difficult to come by. Sheldon's site has a good explantion on the idiosyncrasies of French bicycles. Good luck!