So, I know most books, stage plays/musicals, etc. get changed at least a little bit if/when they're adapted for the screen, but I was actually a bit surprised by just how different Milos Forman's movie adaptation of Hair (1979) was from the original musical (first performed in 1966). Most significantly, in the original musical, Claude and Sheila Franklin are already members of the main group of friends who make up the ensemble. But in the movie, the main group of friends is just Berger, Woof, Hud, and Jeannie (with peripheral extras) - Claude enters the picture as a kid from rural America who's already gotten his draft card (again, unlike the original musical, where this happens midway through), and is kinda adopted by the group during the couple days that he spends passing through New York City. And Sheila Franklin is just some random rich chick from New Jersey who Berger decides he has a crush on (or some such) after seeing a picture of her in a newspaper. And the ending of the movie is incredibly different from the stage musical, as well:
In the musical, Claude gets his draft card, is initially going to burn it, but gets cold feet, and ends up dying in Vietnam. In the movie, Claude never really even considers burning his draft card, and goes off to training, but in a semi-convoluted mini-plot towards the end, Berger disguises himself as Claude so that Claude can sneak off base and visit his friends one last time, only for Berger to end up being mistaken for Claude and shipped off to war instead of Claude. So in the movie, Berger dies in Vietnam.
So, does anybody have any idea why such big changes were made? I totally get that dialogue, song lyrics, etc. get altered and cut all the time to make a story more suitable for the big screen, but these changes just seemed unusually large - e.g. full-on messing with the plot and completely changing characters in many ways.