Well, on of the reasons Murphy isn't performing till tomorrow night is because the producers (as well as herself) want her voice to be well-rested and avoid any problems so she can be on in all performances leading up to the Tony awards.
And it's known that Morrison played Lenya in both workshops and Donna Murphy was approached when it was decided to bring the show to Broadway, not before.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"