The most obvious difference is the removal of Babkak, Omar and Kassim as narrators in the show. In the previews, they had frequent Arabian Nights reprises and did frequent 4th-wall breaking scenes in front of the curtain. I believe there are now additional scenes (that don't break the fourth wall) where those reprises and 'curtain' scenes used to be.
Another change is the removal of Call Me a Princess for Beyond These Palace Walls.
Something I noticed was that, in the earliest previews in Toronto, Kassim, Babkak and Omar were not thieves, while Aladdin had turned to a life of crime after his mother's death. In fact Kassim chastised Aladdin rather harshly for turning to stealing. (Even going so far as to say "What would your mother think?" right after Aladdin is almost killed by guards who say he is such a worthless street rat it isn't worth killing him. Way harsh, Kassim.) In the preview performance I saw, their attitude towards Aladdin's stealing wasn't so harsh, and Aladdin himself only stole specifically to feed a beggar woman, in contrast to the earlier preview and what I think is now on Broadway, where he steals for himself/the group (Broadway) and gives the beggar the food after he sees she is poorer off. In the preview I saw, he even said something like "sorry it took so long!" at the end of One Jump Ahead.
In the earlier Toronto previews--I heard this was changed by the end of previews--Aladdin and Jasmine ride off on prop camels that say "Just Married" in the end. They now come out on the magic carpet.
Didn't they begin previews on Broadway without "Call Me a Princess" and the sidekicks as narrators? I believe "Call Me a Princess" was cut near the end of the Toronto run and the book was reworked minus the narrators between Toronto and Broadway. Neither of those are changes that were made during previews on Broadway which is what the OP asked.