I think it would depend on the circumstances of their acquisition a lot. 17th century France had the capasbility to take in territories without changing much about them. The people would stay within their familiar system, but their new overlord would be the French king. French would eventually replace Latin as the language of law and Meissnerdeutsch as the language of educated discourse, though there would, of course, still be books circulating. In the end, the Rhineland would look like OTL Alsace-Lorraine, with local varieties of German spoken widely, but French as the language of the ruling class and bilingualism widespread. In time, they'd be as French as any inhabitant of Montbeliard or Tolouse.
The political dislocation would be limited since the territories would technically continue to be part of the HRE and the old system allowed for pretty wrenching changes in the course of heredity, too. People would be used to that, and it would be no worse than what they know. Most local laws would stay in place initially, but be harmonised to the French system (to the extent you could talk of a French system) in time.
If the French absolutist state of the 18th century ran the conquest and tried to reorganise the newly acquired territory, though, I would expect much greater resistance. For one thing, Louis XIV did not make friends with his wars, and people had long memories. For another, this would basically relegate the entire German-speaking population to second-class subject status in one blow.
The choice between Republican France and militarist Prussia, unfortunately, would not be open to the locals (though it'sds be easy to make). It would be the choice between an unpredictable, pretty tyrannical but peaceful and familiar statelet versus and intrusive, exploitative and warlike, but powerful and fairly modern absolutist kingdom. Religion would most likely not be a major issue given the Upper Rhine was majority Catholic.