Newsies was also not originally supposed to be a musical. Disney decided late in the game to turn it into one after the success they were having with their animated musicals at the time, so they only had about 4 months to write the music
and teach their main cast, most of whom weren't musically trained, how to sing and dance.
That's how we ended up with a clearly uncomfortable Christian Bale singing and stomping around pretending to ride a horse. He was cast before the movie was a musical and was somehow cajoled into staying. And the lyrics of that song throw his character's entire story out of whack too. At least he was cute.
I still love the hell out of that movie. Just watched it again last night. I even think that if wasn't a musical, it never would have achieved the cult status it has today, because the last hour, that has fewer songs, is where the movie starts to drag.
What's great about the new stage version is that they've re-written almost the entire script and re-worked the songs so that it flows better as a musical now. The "Santa Fe" number is totally different (Jack is now an artist instead of an aspiring cowboy, so no more horse-riding choreography), and it's actually a highlight of the show.
Incidentally, I read somewhere that the stage version has already grossed 5 times more than the movie did in 1992.