Recovery Mode > Disk Utility has an option to convert drives to APFS, including the boot drive. I tried it, and the resulting drive was no longer bootable. This with a FileVault 2 encrypted HFS+ SSD boot drive. So don't use it at this point, unless you're prepared Time Machine your way back.
I was able to later opt-in to APFS by re-installing High Sierra Public Beta 1 from the original App Store release. Just check the box this time. Since there hasn't been any delta updates yet, the reinstall over an otherwise working setup should be pretty harmless. Mac running with APFS now.
The installation app gets deleted during the first setup, but it's available in Time Machine from Sierra era, if you made one before installing. Or just download it again from App Store.
"Not bootable" means: Recovery Mode > Startup Disk complained about not being able to "bless" the newly converted startup disk. Consequently the startup process couldn't find a boot drive that could be mounted. Checked this by booting in Verbose Mode.
I didn't study it any further. I just wiped the drive and pulled back a working Time Machine copy. In this beta the "convert to APFS" manually (on a boot drive, via Recover Mode > Disk Utility) just didn't work for me. But after restoring from a backup, re-installing High Sierra on top of everything with the conversion checked, did get APFS enabled.
Do you think there was a way to bless/charm/summon the manually converted APFS boot drive to be bootable again? Just in case someone steps on the same tripwire.
I found that in some situations, the Mac was able to start from the volume despite the reported non-blessing. Somewhat inexplicable (sorry) but that's what happened. I wrote about one such experience but my link to the writing was removed.