I am just guessing here because i have not done it.
- You can create a shape key and sculpt the new head in. Not sure about the detail/resolution of the mesh
- Copy the rig to the new mesh?
- Multiresolution modifer will defenetly help here. Set it up with the base mesh and copy the rig with the mesh. On a higher level you can sculpt in the new face.
I think thats how the human Progress product is doing it but you can look there he explained it somewhere…