According to the Reddit thread, the film eliminated some songs and changed the ending so Evan is more the "hero" of the story than in the musical.
I do not understand the negativity about the casting of Ben Platt. I do find the fair in the film trailer unflattering. But I don't think he is THAT MUCH TOO OLD to play a highschooler. One of the reviews went as far as to suggest it made his scene with Zoe creepy. Kaitlyn Dever is 24 yd. Platt is 27. It's ridiculous to call that creepy. If the reviewer really feels that way based on the "looks" of the two actors, I think the reviewer should reexamine his own bias. By the way, why didn't anyone say Julianne Moore "looks" too old to be having a teenager son? I haven't seen the film and don't think the musical is without its problem. But I always suspect the reaction against Platt is partly a misplaced sympathy for all other replaced Broadway actors.