It took Disney nearly 70 years to bring its most successful animated picture, "Frozen" to theaters.

Walt Disney tried adapting Hans Christian Andersen's "The Snow Queen" as early as the 1930s; however, he could never get the story quite right.

After the studio revisited the story in recent years, the resulting film was an instant hit in theaters making more than $1 billion to date.

But a new special on ABC, "The Story of Frozen: Making a Disney Animated Classic," reveals that one big change in the film could have cost the Mouse House its billion-dollar movie.

While the film revolves around two sisters, Queen Elsa and Princess Anna, early sketches show the two weren’t originally related. Anna was a peasant who asked an evil Snow Queen to freeze her broken heart.

Elsa's character looked completely different. She originally had light blue skin and short, spiky blue hair. She even had a coat made out of living weasels. Sketches show how Elsa's character changed dramatically in early production. ABC/Disney

Early sketches of Anna and Elsa before they were rewritten as sisters. ABC

"Elsa was going to be the complete antagonist," says director and screenwriter Jennifer Lee in the book "The Art of Frozen." "They kept calling her the 'villain.' But there came a point where we said, 'We can't use that word anymore.' You care about someone who's been forces to hide who they are. Elsa's not a villain, she just makes some bad choices because she's in a very difficult situation."

According to the ABC special, a big push for the character's redesign came from songwriter husband and wife duo Robert Lopez and Kristen Anderson-Lopez who were working on a song to go with the character.

"We were still writing a villain song and we started getting into the head-space of what you'd feel like if you were that isolated," says Anderson-Lopez.

"We knew in this moment that she would go through a transformation," said Lopez. "From repressed to letting her powers out ... trying to get away from society and be who she really was."

Elsa tries out her magical wintry powers during the big number, "Let It Go." Disney/YouTube

From there, "Let It Go" was born and Lee knew she had to rewrite the film's script to accommodate for the change in Elsa's character.

"The issue that kept coming up is, 'What do Anna and Elsa have to do with each other? Why does it matter, and how could it matter more?'" says Lee. "Someone in the room said, 'What if they were sisters?'"

Women from Disney Animation collaborated together in what was called a Sister Summit where they all discussed their own relationships growing up with sisters in order to strengthen the characters.

"Elsa became more the yin to Anna's yang, as opposed to the bad guy," Lee recalls in "The Art of Frozen."

Princess Anna and Queen Elsa in "Frozen." Disney In the end, it was worth it. Instead of an Evil Queen, Elsa's character took on a more symbolic nature as a misunderstood individual who others could identify with.

And instead of delivering a predictable fairytale ending with the power of "true love's kiss," "Frozen" flipped Disney's traditional princess model on its head by emphasizing the importance of family over any potential romance. That theme and the bond between sisters Elsa and Anna helped make it a universal film in 2013.