Sad to admit, I am doing this from memory...
Daphne Rubin-Vega left on 4/5/97 (Marcy Harriell replaced her)
Idina Menzel left on 7/1/97 (Sherie Rene Scott replaced her)
Jesse L. Martin left on 7/13/97 (Michael McElroy replaced him)
Taye Diggs left on 8/31/97 (Jacques C. Smith replaced him)
Adam Pascal left on 11/2/97 (Norbert Leo Butz replaced him)
Fredi Walker left on 11/2/97 (Gwen Stewart replaced her)
Anthony Rapp left on 1/4/98 (Jim Poulos replaced him)
Wilson Jermaine Heredia is the only one I can't remember for sure. It was definitely sometime in November 1997. He was replaced by Wilson Cruz.
It is actually incorrect that none of them reprised their roles.
Wilson Jermaine Heredia rejoined the Broadway cast in 2002 or 2003, I believe. Not entirely sure. Anthony Rapp did a two week run with the Angel tour in Chicago after leaving the OBC. He closed that run on 1/25/98. Daphne Rubin-Vega did a stint in the Angel Tour in Los Angeles in early 1999.
Anthony Rapp, Adam Pascal, Jesse L. Martin and Wilson Jermaine Heredia all reprised their roles in the original London company as well.
ETA: I believe Daphne left the show because she got a role in the film "Wild Things" with Kevin Bacon, Matt Dillon, Denise Richards , Bill Murray and Neve Campbell. I don't recall there being any bad blood as a result of her leaving and she did press for the show and rejoined the Angel tour later on. Reportedly, they were all so family oriented that everyone from producer down to stage hand were all really thrilled for everyone's success.
Updated On: 12/30/05 at 09:21 PM