The above solution may work, but it is scrappy and should not be dont that way.
You should have taken duplicates of your original head prior to skinning the rig. This ensure zero issues with blend shapes.
My advise would be export your weights, go back detach the skin do the duplicates (all must be of the original head, not copies of the duplicates) rebind the skin to the rig and import the weights.
Chris (formerly R@nSiD)
When the power of love overcomes the love of power the world will truely know peace - Jimmy Hendrix
Winner SM VFX Challenge 1
3rd Place SM SteamPunk Challenge (May 2007)