He's got an Emmy for his comedy work, and a lengthy list of guest roles as a dramatic actor.
It happens. Contracts end, actors move on, people die, or maybe it's just time for a change. No matter what cause, in the course of producing popular TV shows, it sometimes becomes necessary to replace beloved characters with new talent. Executing a character transfusion is a delicate procedure, but when it works, replacement characters can give old shows new vitality. It's not that the originals were bad; it's just that the replacements are sometimes even better.