Secondly, each of the three Orders has a slightly different system for Conclave appointments, so they probably would not automatically allow the newcomer to their Order to take a vacant Conclave seat in their Order without passing muster first. The Red and White Robes are especially unlikely to allow someone switching Orders to simply stake a claim to one of their Conclave seats.
It is probably easiest to switch to the Black Robes this way, as all you would need to do would be to kill a sitting Conclave member of the Black Robes (this would have to be done outside the Tower of course) and then claim their seat. Of course, you yourself might be subject to challenges, and the leader of the Black Robes might decide you're a threat and try to destroy you himself.
