"THIS IS MY DREAM YOU GUYS MY LIFE LONG DREAM AND ITS HAPPENING," the Supergirl and The Detour actress tweeted.
Benanti is to headline the transformation tale from Oct. 23 through Feb. 17 at the Vivian Beaumont Theater in New York, the show's producers said in a press release.
The cast of the classic Lerner & Loew musical also includes Harry Hadden-Paton, Norbert Leo Butz, Rosemary Harris, Allan Corduner, Jordan Donica, Linda Mugleston and Clarke Thorell.
Ambrose will play her final performance Oct. 21. She is leaving to begin work on M. Night Shyamalan's upcoming Apple series.