We're at High Sierra beta 9, and it still doesn't seem possible to convert an HFS+ Journaled boot drive "in-place" to APFS.
I tested this with a totally fresh install using a late 2014 27" Retina iMac, installing onto an external SATA LaCie Thunderbolt 2 drive that I partitioned into three parts: 500GB for Sierra, 500GB for High Sierra, and 1TB for future use.
While I was able to install the latest High Sierra 10.13 Beta (17A360a) on one of the partitions, I could not get the option to convert that HFS+ Journaled partition into APFS.
Supposedly, it can be done "in place" by booting to the 10.13 Recovery partition, starting the Disk Utility, and using the menu options to "Convert to APFS". However, when I did that those options are grayed out when a partition containing an OS is selected. Interestingly, if the volume DOESN'T have an OS, then that option is available
Awfully hard to test our software on a new file system if I can't convert the boot drive to the new file system…
UPDATE: I was able to erase one of the external HDD partitions, reformat it to APFS, then install the High Sierra beta from the Recovery Partition. It installed, it booted, and it ran through its setup processes. For a very long time, it seemed it was doing something to the internal Fusion drives — and I didn't see them mount on the desktop, but I think maybe Spotlight was trying to index them. During this time, neither Safari nor System Preferences would open ("Application could not be opened)". However, eventually, things settled down and I was able to eject my internal Fusion drive, and I was able to do things again. So we'll see.
Best Answer
Since you have space to spare: convert an empty partition to APFS, then install 10.13 on the APFS partition. Or use SuperDuper or somesuch to copy your existing 10.13 installation to the new partition.