I think that Terra is Xehanort because I think you play as him against the final boss (probably Master Xehanort) in radiant gardens you have to lose and then he gets nocked out and you play as someone else when the other person defeats the boss Aqua and Ven go over to him and find out he lost his memory. The see Ansem coming and run away. When Ansem asks him his name all he can remember is the name Xehanort.
Another thing to back this up is in the teaser trailer at the end Terra's eyes turn yellow the same colour as regular Xehanort