DjC and NeoCloudStrife:
Okay this theory was created by me and NeoCloudStrife (He's from another forum) I am not going to go into too much detail but I will get to the main detail.
In the Birth By Sleep Trailer Master Xehanort said "His heart is getting weaker" well I think that Xehanort had become corrupted into darkness then he does something to Ven's heart making him lose his memory and makes his heart weaker, the more weaker it will get, he will fall into darkness. Well Terra needed to find someone who can be the keeper of his heart (The next person to wield the Kingdom Key KeyBlade). He decided it would be Riku. Well at the end of the game they get defeated and all fall into darkness but the person who will wield the Kingdom Key will be the person to have Ven's heart. When he chose Riku he expected to meet him in the future. But then since Riku fell into darkness the next wielder was Sora, this is when Sora became the keeper of Ven's heart. Now at the end of the game Sora slays himself with the "KeyBlade that unlocks peoples hearts" well when he released all the princesse's hearts I think there was more to it. Ven has half of heart because one half is Sora's the other one is Ven's (In our theory) So Ven became a Twilight Nobody. And since Human Nobodies have anagrams with an X in their name Ven's name was changed to Sora with an X = Roxas. So once he reunited with Sora they became one again. Oh and this is why I think Terra in Kingdom Hearts 2 Final Mix said "You aren't the one I have chosen?!". Because he was waiting for Riku.
This was added my me:
After you deafeat Roxas in KH2FM, there are 2 cutscenes that in KH2 they didn't show it. One was when-in this case-Ven gave them his new name. When Riku gave his new name.
Also, Axel mentions to Roxas: "You have a Heart, don't you?"
After Roxas mentions also this: "Im Sure Sora'll find the anwser.... because he's me"
In these cases This like joins in what NeoCloudStrife's theory says.
Ohh and i almost forgot. There is another thing to that Axel says: "We first met the day you got your new name."
Cutscene: KH2 Final Mix: Remembrance (English Subs)