The technology might be available some day to remove the layers without damaging the picture. Probably a lot like a CT scan?
I think that’s what has been done... great, now I’ve got some webbing to do...
http://www.google.com/search?q=xray+of+leonardo+painting
http://www.telegraph.co.uk/technology/3293799/X-ray-reveals-secret-of-Leonardos-Magi.html