Why would it kick you out of Duke if you become Ruler? It doesn't break the Chain of Hierarchy so there is no conflict. You can be Ruler and Duke at the same time. You just can't be Lord/Knight and Ruler without being the Duke as well.
I can verify, from personal experience, that you do not get kicked out of your position as Duke upon becoming a ruler through election. In addition to what has already been stated.
Edit: I suppose you could then change the realm name, once you are ruler? And Flag, Icon etc.
