And that's a "radical change"?

Rowling's readers know her magic system work, and are used to get thrown new expositional tidbits about it right in the middle of the action - e.g. that fyendfire could also destroy horcruxes and Hermione knew this but it's barely controllable etc.

In a movie, when you've just spent precious screentime showing that the characters found out the basilisk tooth from movie 2 could destroy horcruxes, and you can just show the diadem being destroyed with the tooth as there's no plot reason for them not to have it anymore, you don't stop the action to introduce a completely extraneous bit of heavy going exposition. It would just be bad scriptwriting.

As for the duel, I haven't seen the movie yet but I understand Harry and Voldermort move elsewhere for their final fight. That's also explainable. The way Rowling wrote the scene from Harry's POV, she could make it about Harry and Voldemort even if it was in the middle of a crowd. If you render this on screen, it's much stronger visually if the two characters are really alone in the shots.