I've been pondering this for a while, and I came up with a few ideas, but none are too satisfactory to me.
Ex 1: The King gets in a spat with the Pope, and makes the new Protestant "Church of France" (like England, and Sweden) He forces the nobility to convert, along with his subjects.
Ex 2: The nobility get into a spat with both the king, and the Pope, and they convert to Protestantism en masse. (like the HRE) They force their subjects to convert.
That's all I got so far. Ideas are appreciated!