Still not possible to convert HFS+ Journaled boot to APFS?

Solution 1:

Awfully hard to test our software on a new file system if I can't convert the boot drive to the new file system

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.

Solution 2:

I converted the hard drive in my iMac (no SSD) to APFS. I had to back everything up and erase the drive, reformatting it to APFS from the network recovery Disk Utility, and then from that install High Sierra. None of this went well from the Recovery Partition.