I know Grease has been added already, but there is more to it. The 20th anniversary 1998 5.1 remix of Grease had all of the songs remixed (like adding reverb and background vocals to a couple of the songs) and added a bunch of foley sound effects. This was the only mix available for 20 years. The 40th anniversary 5.1 remix restores the original sound mix and sounds far better than the previous 5.1 mix.
That’s been talked about a bit here.
https://originaltrilogy.com/topic/The-Audio-Preservation-Thread/id/15892
This thread refers mainly to visuals.