Titanic was certainly no different. Most (all?) of the changes made between the late March, early April previews were for the better, making a tenser, tighter, more effective show.
This page, however, demonstrates some of what you might have heard and seen if you saw a preview early in the run.
From the Playbill
--Songs that were cut in their original running order.
Barrett's Song
--The altered words to Barrett's Song and two little scenettes that were cut.
Charles and Caroline
--The cut scene leading up to "I Give My Hand".
The Missing Binoculars
--Fleet and Lightoller talk and sing about the missing binoculars. Taken from "No Moon".
The Complete Loading the Lifeboats Scene--A Work In Progress
--A drastically different view of the loading of the lifeboats.
The Millionaire's Deck Scene
--The four millionaires talk to Etches and sing about their "great crime".
The Third Porthole
Scene
--The third porthole scene currently in the show, only different--features Murdoch, the Captain, Etches, and Harold Bride.
Robert Ballard's Monologue
--Titanic explorer, Robert Ballard (played by Michael Cerveris), had a part in the musical. This is what he said.
Tidbits Lost
--Snippets lost in previews. Little views of what didn't survive.
Read a review of previews
--From Broadway/6174, an interesting review that discusses what was altered.