Tangled changed so much from inception to release that it’s really hard to separate it into stages. Even in the final year of production after animation had started, they were still making big story changes.

The original story of Rapunzel is, like most German fairy tales, quite a dark story by Disney standards. I would assume at some point much more of the original story was in tact, but they moved away from that as they usually do. From what I know of “Rapunzel: Unbraided”, it was never really all that dark. If there were a darker version of the story in the works, it was probably toned down early on. 

One thing I know for sure though is that visually, the film was, in a sense, originally much “darker”. In the first version of the film they were attempting to mimic the style and color pallet of Jean-Honore Fragonard’s “The Swing.” I distinctly remember being extremely excited when they announced that plan because I’ve always wanted (and still do) to see a movie that looks like a painting.

The concept art might be why people say the film started out dark.

They moved away from this style I believe around the time Glen Keane had to step down as director. The new directors wanted to go for the classic “Disney” look. They even went to Fantasyland in Disneyland for inspiration.

I know I sort of skirted the question, but I figured I would chime in with what I know. Everything I’ve said is speculation based on concept art and what stories I’ve heard about the production. Anyone should feel free to add more precise info to this if they have it.