There were changes made, there were, as in the original book, plot holes big
enough to push a planet through. The story is silly, the action even more so. It took the original story as an outline, and added a lot more. Just as well, Princess of Mars is a pretty terrible book. ERB really didn't know what he was doing when he wrote it. He became a better writer later.
I don't care. This is the first movie I have ever seen to get it the genre right. The sense of fun. The sense of discovery. The sense of adventure. It's a marvelous movie. I understand that it has not done well at the box office. I hope that it does well enough that they make the other two.
I loved this movie. Go see it. Get your popcorn and your drink. Sit back and let it entertain you. Do not expect to be moved intellectually. Expect to have fun and you will love this as much as I do.