This happened at least twice with "If/Then," an ailing Menzel doing act one and then Burns taking over. I was at the now semi-legendary "Sunset..." Saturday matinee when Buckley did act one (rather brilliantly, at least vocally; the voice was there) and then Karen Mason did act two.
It happens, doesn't it? I'm moved that they try to push through, and then finally cannot and cede the role once begun.
"I'm a comedian, but in my spare time, things bother me." Garry Shandling