Alright, so when I was browsing the internet a couple days ago, I was watching some of the secret trailers from kingdom hearts 2. Then at the very end of it, I realized something; after master xehenort reveals kingdom hearts and terra looks up in anger, I see his eyes go the evil orange color that both master and the regular xehenort. Also if you look closly at terra;s hair, it greatly resembles xehenort's hair. So what I'm thinking what happened was terra was not able to keep down the darkness within him self, forgot everything, then was found by ansem, and the only thing he could remember was the name xehenort.