So my High Sierra boot partition disappeared after HS GM install which is when I discovered there is no option to install without APFS. Well, that maybe the future but I don't want it. So, I removed it.
My setup is a dual boot with Win 8 (I use it about once a month, by accident). My Mac boot disk is disk0s6, 70GB, and I have my user data on disk0s8, about 300GB, not the boot disk. This division allows quicker restores and less-dangerous upgrades as the data is always safe. You may need far more than 70GB depending on your applications so think about it before you do that split.
I mention that because you have to destroy your boot disk in order to revert to HFS+; having to restore all of your data every time is both tedious and a little more dangerous than having it safely housed away from what ever APPLE has decided is best for us this week.
So here are the steps:
1) Make a local USB Time Machine backup of your boot disk; you can use a network version (make sure it is up-to-date!) but the restore will take HOURS.
2) Install the High Sierra GM onto a bootable USB device
3) Migrate your previous backup onto the new install (if your USB is too small, do step 4 before step 3)
4) Boot into Recovery; using DISK Utility, restore the USB install onto your HFS+ boot partition.
I'm staying on HFS+ as long as possible - the world is not ready for APFS and way too much stuff is affected without good alternatives.