No. You cannot change regular lenses to photochromic ones without replacing the lenses. Because there is not a coating on the photochromic lens but something within the lens that cause the color to change. So you must replace with new lenses if you do want photochromic lenses after your regualr lenses have been made.
No,you can't. The photochromic lens is not simply a coating on it, but has special chemical material in the lens. So there is no way to change the regular lenses to photochromic. You need to buy a new pair.