Voule ton Ellinon
It requires Greece to have a plurality of at least 3 and prestige of at least 1. On top of that Greece needs to be an Absolute Monarchy. Those requirements are fulfilled at the beginning of the game, and the decision can be taken right away.
It changes voting laws to weighted wealth, the government to HM's Government and lowers militancy by 2. Despite the fact that Greece can take the decision right away, it is wise to wait until at least some militancy has been generated.
In the game files the decision is called Helenic Parliament.