I think both HFS and APFS will be supported — at least in High Sierra. However, it is possible APFS will be mandatory for the boot volume and Recovery Drive.
The conversion was absolutely transparent and completely unnoticeable in iOS 10.3. Hopefully it will be equally transparent in High Sierra. I will let you know as soon as the High Sierra beta is released to beta testers.
If we knew what it was we were doing, it wouldn't be called research, would it?
