Lol, they're the same body but they both have a different outlook on life. Ernesto is a very naiive boy who doesn't quite know what he wants in life. Che, on the other hand knows exactly the meaning he wants to place on his life, who he wants to represent, what he wants to do. You could call it a journey, but i think its just a whole transformation of him as a person. To call it a journey would be doing an injustice to the enormity of the situation and what he became.
So, same body - different personas. Yes, I've been analysing it VERY closely.