Not sure if this will help you out much or not... but, if you create a duplicate of your head pre-modification, and apply your blend shapes to it, you can then apply that duplicate head as a wrap deformer onto your modified head. Depending on the modifications, this may cause problems. Worst case, you can use this method 1 head at a time to more quickly remake your blends on new copies for each of your old blend shapes. (I use this method to make 2 symetrical facial blends (1 for each side))