The Pianist had a few VFX changes in the 4K release, most notably a scene with a burning building and also some bombed out buildings.
https://caps-a-holic.com/c.php?go=1&a=0&d1=18595&d2=18592&s1=219209&s2=219134&i=10&l=0
https://caps-a-holic.com/c.php?go=1&a=0&d1=18595&d2=18592&s1=219202&s2=219195&i=17&l=0

They did this as the film was mastered on a 1080p DI (not even 2K) and looked poor even back then. (A post on Blu-ray.com mentions that someone saw a 35mm trailer for it as part of an arthouse cinema screening of later Adrien Brody film The Brutalist, and it looked bad even on 35mm, like switching to a DVD.)

While going back and rescanning the negatives in 4K for a film mastered on a 1080p DI is pretty admirable (for one thing, it’s something I wish Tarantino did with the Kill Bills), it’s also pretty revisionist at the same time. If The Pianist was made five years earlier, there wouldn’t be this problem.

Releasing films shot on 35mm but mastered on a 2K DI really is a “pick your poison” scenario - do you simply upscale the DI (easiest way and preserves the director’s and editor’s vision but doesn’t look as good as a native 4K scan) or find the negatives and rebuild the film from scratch? (can result in a better looking image, even if The Pianist was DNRed, but costs a lot of time and money, there’s a risk of some elements being lost and is also inherently revisionist.)