This is something(among other things) that has been bothering me lately. There are three known instances of a person willingly giving up their heart,and two that we actually witnessed on screen. However one of these is not like the others. A:when apprentice xehanort cast off his body, his heartless was a bodyless form in a brown robe,and his body was a nobody. B:when Sora released his heart (and kairis) his body instantly vanished(appeared in twilight town) and he (his heart) became a shadow. But C:When Master Xehanort cast his heart into Terra, his body just completely disappeared,while his heart went into terra. But why was his case different? Kairi's body was comatose when inside of sora but didnt completely vanish. Or maybe the body just died because it knew that terra was its permanent replacement(think alphonse in the gate,FMA). If this is the case though,it makes no sense for MX to reconstruct as his old body, unless Square Enix was holding off on using terranort until KH3 something.