Man of the Year (2006) looked like it was going to be a lighthearted Dave-esque movie about a comedic everyman president finding common sense solutions through his hijinks. It pivoted pretty quickly.
Highlander: Endgame, original trailer showed the villain, Jacob Kell, as some kind of sorcerer. Suffice it to say, they removed all of the “magic” in the released film. The trailer can still be seen with a quick YouTube search.
Jurassic World Fallen Kingdom had a major part of the plot completely left out of the trailer, which I appreciated, but there were multiple suspenseful moments with the resolution in the trailer which was a bummer.