Uhh. I made one of my vassals the Duke of Volhynia after taking it because he's a genius and things, and it turned into a Republic. Now he's got the 'wrong government type' tag. He was a mayor or whatever the Polish mayor title is in Prince and the Thane, I was unaware this was going to happen. Is there any way to return it to being a duchy or is that just the way it goes?