>>2
Yeah but all cells have the original DNA stored within them. Assuming that the cells could be tricked in reverse with some magic beam, it would result in making the subject go younger. Reecently there was someone in the news who got rid of her illness because of an transplant.
http://news.independent.co.uk/uk/health_medical/story.jsp?story=630861

Anyway, I think I found the answer to my question: the mass would stay the same. Because evacuating the extra cells would pose a major logistic problem to the body. In the end he would be a giant baby - same total mass as the original. (Eeeek)