Compared to some anime adaptations, One Piece doesn't change much. Unlike, say, The Promised Neverland or Tokyo Ghoul, the plot stays more or less true to the manga. There are several filler arcs, but none of those change things dramatically. Of the changes that do occur, many are related to individual moments that get a little too intense for the anime's intended audience.
The anime adaptation contains quite a bit of censorship, especially early in the series. Zeff still loses his leg in both versions, but it's only in the manga that he cuts it off himself and eats it to survive. Other changes are not as drastic - for example, Vivi's top was considered a little too risque for the manga and was replaced with something else.