I too have been interested in Shape Shifting and after much researching I have found that in the Navajo traditions, shape shifting exists but it has been associated with very evil behaviors and practitioners have to commit to basically a black witch way of life, often having to kill someone close to them, robbing graves and cannibalism. It is thought to be a very evil thing in the Navajo belief systems and its practitioners embody all the negative attributes of greed, jealousy, coveting, manipulating others, murder, etc... I have also heard of other accounts where before colonization, shapeshifting was not always associated with evil but of protecting the tribe. During the European invasion of the Americas, what appeared to be animals would sneak into camps to discern the medicine of the people in the camps. These apparently were shapeshifting scouts doing recon for the protection of their communities. Since the invasion however, at least from what I could learn about the Navajo, the shapeshifters, or skinwalkers, are now black witch loners that are feared by their communities and that curse members of the community they are jealous of or that have things they want or that they perceived have looked at them the wrong way. It could be anything. It doesn't take much. I'm no expert though. There may very well be positive medicine people that also have this power. I'm not sure how they are able to do it but I don't think it's painful. It's one of those magical things that just happens. It's definitely not science, atom by atom shifting. It's a magical transformation. That seems true from what I could find.