Revealing mistake: When the oldest son is being pursued by the unseen entity whilst the family is seeing the priest. You can see a crew member in green pants and brown shoes to the left as the camera perspective turns upside down.
Revealing mistake: When Sonny is in the attic with the priest, as he comes flying in the air towards the priest, you can see the safety wire attached to his back.
Answer: It is unknown why the changes were made, only theories as to why. There has been speculation that because they film took so many liberties and based some of the events (like the incest) on rumors rather than proven facts, that the names were changed to avoid trouble. There's also the fact that the films becoming increasingly fictionalized with each installment, hence they might have changed the names to indicate that the film was only loosely inspired by true events. Finally, there has been some fan speculation that the movie is actually a sequel, and we're merely seeing the DeFeo crimes being recreated in another family, though this is a bit shaky.
TedStixon