How did one song completely change the story of Frozen?

The answer

In early drafts of Frozen, Elsa and Anna were not sisters. Elsa was a ruthless villain - the Snow Queen - with black spikey hair and a dark wardrobe, who planned to freeze Anna's heart and conquer her kingdom. The entire narrative was a conventional hero-versus-villain fairy tale. Everything changed when songwriters Kristen Anderson-Lopez and Robert Lopez presented their demo of 'Let It Go.' Co-director Jennifer Lee later told the New York Times, 'The minute we heard the song the first time, I knew that I had to rewrite the whole movie.' The song was so emotionally powerful that making Elsa a villain no longer fit. The team restructured the story to make the sisters the emotional core, transforming Elsa into a misunderstood protagonist. Frozen went on to gross $1.28 billion worldwide.