How would the protestants convert 50% of the population? IOTL the maximum the hugenott ever attained was 10% during the 1560s, and I don't see how the could get more influence with Catholicism still being the state religion - and use the advantages of this position to fight the spread of Protestantism.
The results would depend on how the protestants would be distributed. If they are concentrated in certain regions, the result would be a split of France into two states after a series of religious wars. If on the other hand they would be evenly spread out the result would most likely be a horrible civil war which could easily rage for decades.