Additionally, some classic stories, such as Cinderella and Peter Pan, have several different musical theatre adaptations.
Depending on how broad you would like to go with this, you could also look into more opera and ballet scores. For example, more stories that have separate opera/ballet and musical theatre adaptations (Faust, Aida, Madam Butterfly, Orpheus and Eurydice, War and Peace etc), scores that were adapted from opera to ballet (Carmen again, The Merry Widow), and the like.
(By the way, random: this won't be news to ballet buffs, but for those interested in one/both of this season's Russian-themed musicals, I just happened across a pre-existing 'Anastasia' ballet which uses Tchaikovsky music for the past tsarist scenes and some electronic music for scenes with the 'modern-day' Anna Anderson claiming to be the lost Anastasia. Interesting. https://www.youtube.com/watch?v=2BuWCQ868bY)