Yes, if there are tickets, they will likely allow you to switch - but I'm assuming you purchased the tix through telecharge/ticketmaster? That MIGHT change the way you have to go about it (but I'm not sure).
However, they won't allow that to happen until it's official that she's out. I'd just stop by the box office at youre earliest convenience and ask them. They'll be able to give you the exact info necess. They open at 10.
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