egamma is correct. A noble who has an oath and gets elected ruler keeps their existing oath and estate. However, once you become ruler you cannot swear a new oath of fealty.
You have to remember that an "estate" is part of an oath of fealty. That means you are swearing to serve and obey a lord who you acknowledge is in charge of you, and has a right to tell you what to do, and you are sworn to obey. These oaths of service all point to the top, which ends up with the ruler. And the person at the top swearing fealty to someone at the bottom just doesn't make sense at all.
