You tell them now. If you dont, you are a liar -plain and simple. Your obligation is to the first show you commit to. IF they cast while keeping in mind your limitations, great.
And Im going to put this out there: ypu dont audition for plays just to be the lead. Your attitude is going to get in your way.
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.