I say that it is theoretically possible to change yourself into an animal (dragons are a different matter), but it is so extraordinarily difficult that there's really no point. Besides, actually turning yourself into an animal would likely cause you to forget your human and become that animal in mind as well. The ability to change back and forth (similar to the animagus in Harry Potter, just for reference) is probably impossible for all but a select few who have inherited whatever genes would allow such (if they exist, of course).
now i'm upset. normally i take transformation people with a grain of salt and am nice to them, but not when it involves Dragons. you CANNOT become a Dragon! first of all they are not of this relm, second you cannot change form physically in this plain! Dragons are wise and powerful, so i see why you'd want to turn into them, but you cannot and i find it insulting. sorry to jump down your throat, i understand it was merely a question, learn from dragons, perhaps there's a spell to change your astral self into one but not your physical self.