For reference I owned the previous Criterion releases of Yojimbo and Sanjuro and I can say with conviction that both sound and picture-wise these releases are noticeably better. The film has been touched up in post to take away some of the damage done over time but you don't need to worry about some absurd George Lucas-level liberties being taken, the only changes made are ones that improve quality of the viewing experience.
Additionally they feature new translations which may put some people off. One of my favourite lines in the first film was translated differently this time around and in my view it doesn't have the same impact. That said, overall, I think they're certainly better and arguing about which version of which line is better is merely personal preference.
If you like Akira Kurosawa films, or just samurai films in general, these are certainly two of his best. If you already own these films and are on the fence as to whether or not the improvements are noticeable I can assure you they are.