Most shows don't know who's in and out until about 3 hours before curtain, since there's always some that don't call in until the deadline time.
You can always ask for your money back if the star isn't on.
I heard this story once - Richard Harris was doing Camelot in Boston and some people in the area wouldn't see the show because they hated Richard Harris. Then they heard that he was out for a period of time and his understudy was fabulous. So, they bought tickets, but by the time they got there Richard Harris was back in the show. They asked for their money back.

I never heard if they got it or not.
Did you ever see something and say, "I just have to have it"?
Sometimes a dream appears unattainable...but for anyone who's on the downside of advantage, and relying purely on courage, it's possible.