Make a copy of the detailed bust, dynamesh the copy to a body, and then project the detail from the original bust to the dynamesh. edit: Actually, just activating the Project setting in the Dynamesh menu should preserve the details from the original well enough. I'd still keep a copy of the original in case you need to…
Just use Dynamesh, it should keep the base, large details of the mesh, and only lose out on the smaller details, just proceed in creating the entire thing, once done, do what Big told you.