What is the biggest most drastic change you've made to your story?

I'm revising my whole story, it may not sound like a lot but I'm completely re writing 7 chapters of work!! And I'm changing the plot, and the main character completely, she went from innocent new girl to troubled teen who has a severe case of cabin fever.. and she's become the type of hero who wouldn't die for her friends…she's become more of a warrior, fight first, check wounds and emotions later…

I'm only just beginning with this story, but recently I discovered that the MC best friend, who appeared to be a cinnamon roll and is a character I never thought he'd do anything bad in his life, is actually The Bad Guy™. I still have trouble accepting this fact, but it makes a lot of sense considering where the story is headed.

While writing a draft for a book I grew annoyed with one of the four important characters who was supposed to be a very charismatic dude who could get the ladies. But he clashed with another character who was a sixteen-year-old girl trained in hunting and self-sufficient who refused to take a back seat and do what she was told (mainly because they hired her as a guide). then a friend suggested a fight scene between the two during which the main character would notice the dudes unstable, fragile ego and kick him out.
Afterwards, the story had a much better flow because the main character now had someone of his own age to talk to who had a different worldview and treaded him as an equal.

Changing the world entirely? One time I made some characters in an already-existing fictional setting, then it kind of became less and less like the source material until I switched it over entirely. It's now becoming an established world with cultures and relations and backstory and everything.
Other than that probably the time when I decided my story needed a villain, because it had none, so I purpose-built one. Somehow they ended up becoming one of the 4 main protagonists. I still don't have a villain.