Laharl becomes a prinny at some point, but I don't remember which timeframe in the Disgaea canon that is (given that there are also multiple "proper" endings to each game, it's a bit vague as to which one is canon in and of itself).
From what I recall, there's a huge war that takes place some time after Disgaea 1 that Laharl is killed in, and that's when he becomes a Prinny (that one ending from Disgaea 1/the anime is not the 'canon' event that turns Laharl into a Prinny). Etna tries to work him exceedingly hard so that he'll become a demon again more quickly, but he still retains his selfishness and arrogance as a Prinny so idk where that goes.
But he's still a demon in Disgaea 2, 3 and 4, so I have no idea where in the timeline the war takes place. I haven't played Prinny: CIRBTH 2 so if more light was shed on the subject in that one, that'd be good.