At left, current Velma Kelly (through Jan. 16) Mamie Duncan-Gibbs with her replacement, the original revival Velma, Bebe Neuwirth. Neuwirth steps back into her role Jan. 18. At right, Marcia Lewis, who plays Mama Morton, with Ernie Sabella (Amos). All four were celebrating the arrival of TV actor Robert Urich into Chicago's cast. Urich, best known for his TV roles on "Vegas," "Spenser: For Hire" and "The Love Boat: The Next Wave," will play Billy Flynn at the Shubert Theatre to Feb 13.