No wonder he's the one who made everyone change their names.
That's...a possible motivation, I suppose. I was under the impression that he rewrote the names with an X in order to "mark" those he deemed worthy with the sigil.
Later when Xehanorts body is left behind Terras heart remain in the body, same as Ventus remained with Roxas and by then he take on the name of his new Master - Ansem....Terra is Xemnas but he was norted, probably when he was trying to plot against Xehanort and trick him
Sorry, I don't think I quite understand what you are saying here. But, um...my personal interpretation of all this Xehanort stuff is:
Terra's body is forcibly given Master Xehanort's heart at the end of BBS. The two hearts clash inside of the body, causing chaos where they are both fighting for dominant control over the body itself. In this state, he is constantly shifting between being the consciousness of 'Xehanort' and 'Terra'. It is almost like the body is acting on impulses as the two hearts duke it out in a mental fight inside, one side sometimes briefly taking control. In the end, this body is stabbed again by Terra in an act of defiance where he tries to forcibly kick MX out and he collapses (as seen shortly after Aqua fights him in RG).
At this point, it is unclear what happened to their hearts, but we do see a scene later where they are chatting in some dark area about how they are still having a "property dispute" about the body. So clearly they are kind of unresolved in who controls the body. My theory on this is, then, that Terranort as a body is now a bit more "cut-off" from the influence of their hearts, because they are having a battle in a more closed-off space. This is why when he wakes up in RG he has absolutely no idea who or what he is. I think he said the name Xehanort because he was at that time a mush-mash of both people. So I really don't think Terra was in control then, or if he was, was trying to purposefully conceal his identity for some reason. But I can see how this could be debatable.
Now, when we are talking about apprentice Xehanort, we have a scene in one of the secret endings that shows him chatting with Braig. Er, rather...Braig chatting TO him about how his memory is gone. Braig is upset that he isn't acting like Xehanort, and is also questioning if he is Terra, but doubts it when Xehanort gives no indication of understanding either way. I think this only proves that while apprentice Xehanort has this weird mash-up of Xehanort and Terra inside of him, he is not being directly controlled by one or the other and it just sort of acting on impulses he feels from the presence of both of them.
When apprentice Xehanort then gets wrapped up in his research and splits into his Heartless and Nobody forms, I also assume that Terra's influence is kind of just chilling inside of Xemnas. This is why we have scenes of Xemnas seeming to remember Aqua and Ven. I don't know if that is "his darkness calling out for the love of old" or if it is just the faint traces of Terra's heart still lingering, but again lingering so far below the surface (because of stabby-mcgee) that he can't really directly control or say anything.
Maybe it is more like, to use another example...in KH2 when Roxas first goes back to Sora, and Sora then wanders out into Twilight Town and meets Hayner, Pence and Olette. He is really shaken and feels both nostalgia to the town, and an attachment to his friends. So much so that when Sora has to leave them he starts crying and honestly has no idea why. It upsets him, because the feeling is so strong but he has no idea where it is coming from. This is how Xemnas is operating, I think. He has these feelings from Terra of love for his friends that makes him "feel" things towards Aqua's armor, for example, but it is entirely possible he has no direct contact with Terra at all. That's why I think he is seeking out his friends.
Maybe this is what you were saying? I'm sorry if I misunderstood and basically just restated your opinion.
As far as why apprentice Xehanort took over Ansem's name...I don't think that was an echo of the Terra/Xehanort situation (I mean, it
could be) but instead it always felt like to me that apprentice-nort took over ansem's name because of pride. Ansem initially lead the research on hearts, but backed down when he realized it was a mistake. Xehanort wasn't happy with leaving it as it was and kept pushing, eventually feeling like he had "surpassed" Ansem. It was a way for him to say "I have done what you could never do / were afraid to do and I will carry on your name in your place." Mostly I get that impression from the scene where they are chatting in KH2 right before DiZ/Ansem blows up.
Xemnas says specifically "I'm carrying on what you yourself began..." so it feels like the name switch was, again, more of a "I am assuming your duties because you gave up like a coward".
It's hard to say where Terra is, exactly... but we at least know Xehanort assumed control over the body briefly (as seen in 3D) when he attacks the other apprentices to make them into Nobodies. This is again why I think the name change aspect within the Organization was a conscious decision from MX to mark them with the sigil.
We also might have to consider with Time Travel shenanigans. Apprentice-Xehanort when he was first turned into "ansem" maybe have seen things in the future at that time (because that was when he 'left his body behind', the first requirement of time travel). Perhaps why then, when he wakes up and starts going on a stabbing spree it's because of what he knows from the future.
Interesting things to think about though, thanks for the topic suggestion.