Genesis 8.1 is a separate figure included with an updated Genesis 8 Starter Essentials (and there is a new PBRSkin sahder in an updated Default resources). DS 4.15 is required to take full advantage.

Genesis 8.1 will pick up most things for Genesis 8 (that's one of the reasons you need an updated DS), but expressions have been reworked - the basic bones are still there, and custom morphs will work, but the old Daz Genesis 8 Expressions (the companion product to the head and body morphs) won't load without fiddling, and that of course breaks add-ons which depend on the product. Said fiddling does introduce issues of its own, and having both the new and old expressions does slow loading, so it's not an unmixed blessing (which may well be why the product is set up the way it is).

The original Genesis 8 is still there and still works as before.