Rough guide on how I converted a head to high poly in Blender. It's somewhat of a step-by-step, but I did leave out explaining some basic tweaking techniques for finishing. you'll want to know how to do some light manual welding, how to snap vertices, and how to fix individual normals in order to do a whole head conversion.
it's not so much a full blown tutorial as it is me sharing my method, there may be other ones and they may be better, but this is what i did. if you choose to try this out and notice anything i didn't mention, let me know and i will update the post
PLEASE NOTE: I haven't tested this thoroughly enough to know if things such as shape keys/morphs will still work. my educated guess is that they probably don't and can't (currently), but you're welcome to give it a try!