Ok, it’s easy to change faces around; it’s one of the first things people do when learning Photoshop.  What separates the good from the bad is the choices you make before you start.  Technically anyone with a couple of hours of experience with Photoshop can do a head swap; it will look crap and probably serve no purpose other than to embarrass a friend or relative.  To do a good one you have to have the right material. The subject’s expression is the first pointer to potential uses; a neutral expression will be boring unless placed in a context where a neutral expression would be extraordinary.  If your subject’s expression is exaggerated then it will be easy to make something good from it.
Mona Lisa may be an icon with her enigmatic smile but it’s still a very boring picture (there’s nothing happening).