Say you're an Irish minor wanting to make use of that +tradition they get. Or really any OPM, but I was wanting to do it for an Irish game. The only ways I know of changing to a republic are Milan and the Netherland's unique events, and letting revolutionary rebels break you. So of course being an OPM would make it quicker. But while I have seen revolutionaries a few times in pre-1500 games, it's usually peasants and they always want a despotic monarchy. Do you need to somehow reduce your legitimacy, or something else, or is it just random?