One of my fave movies, and the inspiration for Man of Steel, underwent a makeover in 2006. Here's the back story: In 1977, Richard Donner was shooting both Superman and Superman II at the same time. When the first film needed more work before release in 1979 Donner stopped working on the sequel when it was more than 3/4 done.
Before Donner could resume production the producers decided they didn't want to use Marlon Brando's already filmed scenes in the second movie, this way they could save a bunch of money. They also wanted a more campy approach. Donner balked, walked and Richard Lester (A Hard Day's Night) was brought in. The new director made numerous structural changes to the script and refilmed many of Donner's scenes, for whatever reason.
About 2/3 or so of the film we saw in theaters was Lester's and the rest Donner's. Twenty plus years later an editor went back to Donner's filmed footage and restored Superman II to something a lot closer to the original script and director's vision.
In Superman II: The Richard Donner Cut, Marlon Brando and Gene Hackman have greatly expanded roles (Brando was cut entirely from the theatrical release) and the result is a much more serious-and violent-film. Christopher Reeves' performance is much bolder as well. The slapstick nonsense in Superman II is happily exiled to the digital Phantom Zone, less than 1/5 of the Donner cut uses Lester's filmed sequences.
One has only to look at what came after for Richard Donner - the Lethal Weapon series for instance - to know who should have directed Superman II to the end. Even the music score by John Williams, culled from work done in 1981 and from the first film, is more pleasing than what was used in 1980. Sadly not everything could be restored, there were scenes never filmed, but it's absolutely incredible that we got another bite of this sweet apple and it turned out so tasty. Rent or buy the Donner cut of Superman II and experience one of the best superhero movies ever made.