no, you cannot change form with magic. it's energy, and it's used to help you with love, jobs, luck, health and so on.
people changing into dragons are myths, legends, a.k.a. stories you tell people to entertain them.
mind you, you could work with dragons, since they're astral beings, you could gain strength from them, but that's about it, you cannot physically become one... maybe astrally, but i don't know, never tried, never wanted to.