Casting Change: Ron Silver is out and his understudy, Tom McGowan, is, at least for the moment, the new star of "La Bete," a Broadway-bound comedy that opens today at the Wilbur Theater in Boston. Gary Springer, a spokesman for the show, called Silver's departure "amicable," saying that the actor's withdrawal was due to "irreconcilable interpretive differences." Whether McGowan will have the role when the play opens on Jan. 31 at the Eugene O'Neill Theater in New York is not certain. The play, written by David Hirson and directed by Richard Jones, takes place on the 17th-Century estate of a French prince, whose court is disrupted by a manic troubadour. The producers are Stuart Ostrow and Andrew Lloyd Webber.