It is extremely hard to go from comedy to drama. There are very few that have ever done it successfully. The longer and more succesful you are the harder it is. The reverse however is not true.

One major example of comedy to drama success is in the movie "Casino" where you have 3 well known comedians doing serious drama acting pulling it off well, IMHO. They were Don Rickles, Alan King, and Dick Smothers. Joe Bob Briggs (TV) and Joe Pesci were in it also and have done a lot of comedy.
(Great movie btw-I go to the Casinos a lot as you all know)