Um why would you assume you have demonic powers in the first place. Also magic can't really transform you into anything.
Magic doesn't allow physical changes. It can impact how we and others feel about and interact with the world, it can change out comes and point things in your favor, but it won't make you a vampire, werewolf or demon.
Magic is really only dramatic when dealing with magical planes and forces directly. I'm sure there are demonic forces on the astral plane and some such, and you could definitely mutilate your soul to become a creature of evil.
I have another question. Why on earth would you want to transform into a being of evil, or expect anything to help you. If you are actually magically talented you might get your wish granted in the worst way imaginable
(TO PEOPLE WHO PRACTICE GEOTIA, EVOCATION, ARE SATANISTS, ETC) I'm aware there is more than one interpretation of what demons are and how they behave. But the person on this post probably has a hollywood idea of demons.
To sum up
You will never get red skin, horns, and cloved feet in our plane of existance. Spiritually it's quite possible. The worst demons are completely human
If you shouldn't want to become a being that takes pleasure in rape, murder, and the destruction of all human hope
You can't transform into anything, this isn't like Harry Potter, where McGonagall can point and her desk transforms into a wild pig
Try looking up older more established branches of magic. I think you might enjoy invocation and evocation.