Is there any evidence that a sorcerer can transform a human into a donkey, and a donkey into a human, by the power of Allah?

There is no evidence that magic transforms realities. The majority of scholars believe that it does not change realities, but rather alters one's disposition, becoming a type of illness. Magic is a reality according to the Sunnis, and practicing it is an act of disbelief (kufr). It affects bodies through illnesses, death, or madness.