Having just gone through a revamped Once Upon a Mattress, I have several thoughts on both sides of the issue. I don't like this current attitude in revivals of "let's fix what's broken," as if the original - and usually dead - authors were too stupid and untalented to repair as best they could while their show was either out of town or in previews. I also dislike the idea of "let's replace the original score with the movie songs, as if the audience will feel safer watching a poor version of a film they liked. I've always preferred the stage version of The Sound of Music to the film: "My Favorite Things" in the show is used to point out the commonality of Maria and the Mother Abbess. Giving it to the film scene where "The Lonely Goatherd" was originally performed turned htat song on film into a Bill Baird puppet moment; those Trapp children didn't just sing, they were virtuoso puppeteers! Speaking of theis show, the Broadway revival with Rebecca Lukewr was really misguided; Jan Maxwell may have been a funny, bitchy Elsa but she sure couldn't sing the high B in "No way to stop it"; did no one care what voice Rodgers wanted? Also, in that misguided revival, the orchestrator added Austrian folk instruments like recorders and zithers, which the original vocal arranger Trude Rittmann disliked; she explained that the Trapps were upper class and would have looked down on "folk" music.
In the last Hello, Dolly!, adding "Penny in my pocket" was poor because it no longer had its setup, so Vandergelder just appeared and sang the number, which was ridiculous.
On the other hand, a lot of older shows, particularly ones written by Herb Fields, tend toward politically incorrect raunch, bawdy material, and ethnic caricatures; given that Fields was a Jew, his script for Fifty Million Frenchmen contained a rather anti-semitic portarait of the one Jewish charactere in the script. I think such things should be cleaned up. but the approach should always be to not throw out what's good.
When I was working on Babes in Toyland for the Houston Grand Opera in 1990, Gerald Gutierrez, who had directed the HGO's fantastic production of Carousel - a problematic show now because of Billy's abuse - said he thought the correct approach to a period show was to adjust only enough so the asudience, if they went home asnd looked up the plot somewhere, thought they had seen exactly that. Now, as to Carousel, which I think is one of the all-time greatest musicals ever written, if you approach the show carefully - as the last ugly revival did - and try to clean up or slide over Billy's uglier side, then the whole theme of his redemption at the end is completely screwed.
I haven't even got to the numerous attempts to save Cabaret or Camelot, but I've gone on enough.