Madeline Kahn left the show early in the run after having missed 10 performances in two months. She was replaced by her understudy, Judy Kaye. At the time, it was reported that the role was too vocally demanding for her to perform eight times a week, and that she had proposed having Judy Kaye play matinees. Judy Kaye took over full time on April 24, 1978. The notes in the booklet for the CD issue of the cast recording give that as the date of Madeline Kahn's last performance, but her last performance was on April 22.
Hal Prince stated in later interviews that she was having trouble commanding the energy to play physical comedy eight times a week, and his statements suggest that he and possibly the writers tried to persuade her to leave. At least a few critics, having attended critics' previews rather than opening night (when Prince says she was wonderful), did find her performance a bit lackluster, although most of her reviews were favorable.
Between the out-of-town opening and the Broadway opening, the role was made more vocally demanding. It may be that this was done at Kahn's suggestion. It does seem that after the Broadway opening, at some performances she sang lower notes in place of some of the high notes that had been added.