Simply put, IMO, I don't see it happening very easily. France had far too much to gain by siding with the Protestants, especially considering her past rivalries with the Habsburgs in previous decades. Besides, increasing Imperial power was not in King Louis XIII's interests: it would a stronger Austria--not something so positive when the war ends, and you have the Emperor's cousin King Philip IV of Spain on the other side of the Pyrenees (not to mention knocking at your backdoor in the Low Countries).