Now, I do believe there are "supernatural' forces positive and negative that can change their appearance to make us feel comfortable with or afraid of them, it is a common belief that humans can do the same and those of us who have been practicing magick for years constantly seem to have to answer this question and we have our answers set on default. It is not possible for such a thing to happen. If it was, none of us here would choose to be human haha this is what we call "Fluff Magic" which is just a waste of time. We don't even bother calling it Magick.
Indeed, as well, I've never gotten why you would want to do sch a thing. Most religion condemns such a thing, and would you want to turn into a vile beast with no control and writhes in agony each transformation, even if it were real?