I can give you a small walkthrough on how to blend one head shape into another:
First you need 2 models of the same vertex order like mckinley said. the best way to do this is do duplicate the model you have already made and move it to one side.
Next you need to move the vertices around on the duplicate model until it looks like the beast you want (make sure you don't add any more parts to the model, all you can do is move vertices!)
when you are happy that your 2 shapes look good you simply select the original model, Shift-select your new model and go to "create blend shape" from one of the toolbars (i can't remember it at the moment, i think its in the animation set somewhere at the top. Don't have maya on this pc, sorry)
now look at your channel box on the right. under the inputs there should be a new channel called something like blendshape_1.
you can then make the old model blend to the new model by changing this value. (0 means there is no blend and the model will look the same. 1 is when the old model will be morphed into the new model) you can click on this value and drag the middle mouse button left and right in the perspective view to see the blend in action.
I hope this helped, i think its mostly correct, but its all going on memory.