Is it possible to darken a pencil drawing to match the colour of charcoal?
I think that there are a couple of ways of dealing with it.


Rework the head with charcoal, possibly using an eraser and add some pencil marks to the rest so that you have a bit of cohesion everywhere 
Leave the skin as it is (with pencil) and use charcoal everywhere else
Use fixative (or hairspray) to knock back the darkness of the charcoal
Use a mist of white spray paint to subdue everything
Take it in stride as a learning experience and make another drawing