The objective is, with a POD no earlier than 1536, make France a majority-Protestant nation.
The most obvious approach is for the Huguenots to spread more widely. If the St. Bartholomew's Day massacre either never happens or backfires - say, the Huguenots have enough numbers in Paris to fight off the mobs and seriously contest the city - then they might have the leadership needed to challenge Catholic control. If Henry of Navarre inherits the throne as IOTL, he might not convert to Catholicism.
Assuming we can bring about a Huguenot France, what might be the impact? French support for Protestant causes in the 17th century would only get stronger. France might outright attack the Spanish Netherlands. This could even lead to an earlier and more enduring Anglo-French alliance. Instead of French Huguenots fleeing to England, we might get French Catholics fleeing to Spain and Italy. The Pope is going to be shitting his vestments at the thought of a major Protestant power with Mediterranean access. The nightmare is a Protestant army sacking Rome. For all intents and purposes, the Holy See just became a Hapsburg possession.