I don't think there is any need to get a new master. Just say that Tyrion is still you master til you reach 700 posts, then you can graduate. Tyrion hasn't said that he's left, just that he won't be around very much anymore. Therefore, he can stay your master for the next 50 posts or so
"Yeah no one likes a smart ass, but we all like stars,
But that wasn't my intention, I did it for a reason..."