The official reason behind the change is because apparently in some European countries (such as Spain) "Moana" is a registered trademark.
Again, that's the "official reason", but I'm sure at least here in Italy the decision has indeed been definitely influenced by the whole Moana Pozzi thing. She's literally the first result that comes up on Google if you type in "Moana".
Personally I think it's a very feeble reason and I dislike the change, especially since Moana sounds so much better but hey, what can you do. I guess it's for the good of the many little Italian girls who in a few months will be searching the internet for pictures of their new favorite Disney princess, lol.