Suppose someone had bought their ticket to see a show - to see someone in a particular show - weeks or months ahead of time, only to find that that person was out of the show when they got to the theatre for their ticketed performance. Well...
I still remember a friend of mine who went to see EVITA during its original (and only) Broadway run. She was all excited to see Mandy Patinkin and Patti LuPone. As she was waiting in the lobby to go in, they started to futz with the cast board. LuPone was out. Patinkin was out. Etc., etc. It turned out that she was about to see the "new cast". Needless to say, she did not have a good night at the theatre that night.
And as has already been mentioned, sometimes a replacement is announced/decided at half hour. And sometimes even mid-show.
By the way, commas, periods and capital letters are always appreciated. Clarity.