I am a user of the program like you, and not a PA, so my knowledge of the background workings is limited.  I have been using DAZ since 2010 though and one does pick up useful tidbits of information along the way.  All HD are morphs, and the morphs tend to follow the morph of the character they are based on, you can dial that basic V6 morph into about any female character.  In the beginning the HD character was sold independently from the basic morphed character, I imagine people got tired of paying for the same character twice, and later ones have the base and HD characters combined.  HD may subdivide the mesh further, I'll need someone with more knowledge than me to clear that up for you.  In any case, you can dial in or out HD morphs anytime you want, they do add to rendering time.

Here is an excerpt from on HD character:

"Liu Yan is a remake of the popular Summer Edition Liu character. Her skin has been completely re-textured to fit Victoria 6 perfectly. She comes along with a custom HD morph for V6's body and face, as well as a bonus HD morph for Mei Lin."

If you want a complete listing of characters with HD morphs, just search for "HD" (at DAZ anyway, doing that here will not pick up anything useful because for a vendor to use DAZ's HD tools it has to be sold at DAZ).

Since HD is morphs not textures, you can use any of your existing textures on HD Genesis 2 characters if you can use them on regular Genesis 2 characters.