To begin with let me just say that Changeling is a good movie. It is well crafted, superbly acted and perfectly shot. Had I not known more about the real life story on which it is based, it would almost be a 5 star movie. However the artistic license taken by the film regrettably marred it for me.
I do not think directors should be allowed to claim their movies are true stories if they do not intend to uphold the facts of their case. Regrettably I cannot tell you which specific facts are incorrect as they are major plot points, suffice to say that if you have no interest in the real story then you may enjoy the film for what it is. A work of fiction.