Pulling a Nelkin
To replace an actor or actress less than one week before the opening of a show by bringing in a completely new person not involved with the show beforehand. Derived from the French Woods Festival season 2008, where this phrase's namesake was plucked from the ensemble of another show and tossed into a leading role in less than 3 days before opening... twice in the summer.
"Did you hear? Maddie's leaving, so Halcyon Days is pulling a Nelkin!"