Posted 08 May 2012 - 08:17 PM
Ansem did not become Xemnas. After the events of Birth by Sleep, Master Xehenort used with Terra and was sent to Radiant Garden by Aqua after he had apparently lost his memories. During his time in Radiant Garden he went into research on darkness and the heart, eventually giving in to the darkness within his own heart, producing Ansem Seeker of Darkness, his heartless. Since he had a strong heart and will before becoming a Heartless, his body became a Nobody known as Xemnas. From that point on they existed as separate beings. Xemnas formed Organization XIII while Ansem SoD traveled between worlds, eventually meeting Sora in Destiny Islands and fusing with Riku in Hollow Bastion before finally facing Sora head on.
Side note: If Ansem SoD had just killed Sora before he had his Keyblade back on Destiny Islands, imagine just how much different things would have been.