there were very few changes in the transition from off to on-broadway. They made more puppets to help with the costume changes made a Few minor ajustments to the set. Some jokes needed to be re-worked, but everything else is pretty much the same.
I know a few of the puppets were redone to have brighter colors for Broadway lighting and that as far as songs go, "Life Outside Your Apartment" has had the most revisions (there used to be a lyric that ryhmed Zabars/ Gay Bar, but then they found out it had been used already in Schwartz song "West End Avenue")
Rick Lyon said in an interview that he wanted to make the current Kate "rounder" on purpose because he felt it would add to her insecurity as a character and provide a sharper contrast between her and Lucy Her also said that he made Lucy with the thought of Praire Dawn gone wild in mind