After having watched "Earth vs. The Flying Saucers" on DVD, and listening to Ray Harryhausen discuss the efforts going into the colorization process, I was in full agreement with him. Harryhausen was a principle decision maker on the movie, wanted to shoot in color, but didn't have the budget. So when the opportunity to colorize it came about, he jumped at the chance, but made sure that colors were as originally intended, especially the flying saucers that were originally intended to be silver would end up silver on the colorized version instead of the yellow that the colorization artist would have colored them without Harryhausen's input. There were no major changes done to the content of the film. But Ray knew that many people wouldn't want to watch the film because it was B&W so he was glad to have the opportunity to make the film acceptable to those who were of the color-only crowd. And the B&W version was still made available on the disc for the purists who insisted on the original release (but the commentary was on only the colorized version).
The particular film restoration house that colorized the film had the policy of insisting that the Director, Producer, or some other major player in the film be involved in the process to try to preserve as much as possible the original Director's intent. They want to avoid the Ted Turner controversy of colorizing against the original artistic intent and producing colors that the Director would have never chosen if the Director had the opportunity to shoot in color. (One major example of this was Casablanca, which, if I recall what I read correctly, the piano used in the sound stage was yellow because that was what caused the particular film stock to capture the piano to the best effect, but would have never chosen yellow if the filming could have been done in color, yet Ted Turner colored it yellow because he was going after the original colors on the sound stage.)
Wire removal, artifact removal, even film grain removal are what I would think are fair game. Maybe redoing special effects, since what would have been acceptable on a 21-in screen reveals its flaws when shown on a 50-in screen, which I find distracting when watching old shows or old made-for-TV movies. Colorization? Get the original Director or someone who still has a stake in the original movie to participate in the colorization process, but don't colorize if those with a stake in the original film want it to remain B&W.
Modernize? Remove anachronisms? Maybe with the original Director or Producer, but I wouldn't want it done with Back to the Future where the anachronisms are part of the humor with the time-traveling teenager. But there was one scene in Back to the Future where Marty's shirt pocket flap is sometimes outside the pocket and sometimes inside when he is talking to his father; I wouldn't raise a complaint if that were digitally fixed, nor would I object to the one spot where Dorothy is dancing on the Yellow Brick Road and for a second her ruby red shoes (which were actually more of an orange to get them to look red on the film stock) were black would be changed to ruby red.
But when a major feel of the film or a significant story point is changed, it detracts. Some films are purposely shot in B&W for the atmosphere, and atmosphere is what some of the old horror and mystery films can just ooze out of the celluloid. (Tell me if you can watch the 1951 version of "The Day the Earth Stood Still" without feeling the paranoia of the cold war era!) The guns in E.T. were a part of the menacing of the government coming in and scaring Elliott and his family, and that menacing was decimated when the guns were replaced by walky-talkies. Whether Han Solo shot first or second changes just how great Han's redemption from evil to good was, and I would even go so far to state that the added CGI for Episodes V & VI gives a feel that some scenes were "enhanced" just to show off the better effects but detracted from the story.
If one is going to mess around with the story, market it as a different cut, or make both versions available on disc. The problem with messing with the story is that those of us who have fallen in love with the original are jarred out of the movie when we hit the spots where what we are watching differs from what we had grown to love.
I can put up with wires, film grain, mat paintings, the cue dot, scratches and flaked-off gelatin far easier than a story change!
At least usually these changes aren't as jarring as the hatchet job some stations have done with some movies just to show more commercials. Yes, one such local station butchered out the famous dentist scene in the Marathon Man and lost the 70th most quoted line in film history: "Is it safe?"
That's far more atrocious than pan-and-scan!