Can you convert a Hard Disk Drive (not SSD) to APFS?
No I would not do it at this time. Apple File System is optimized for Flash/SSD storage, but can also be used with traditional hard disk drives (HDD) and external, direct-attached storage.
You can use Disk Utility to convert external volumes from HFS+ to APFS format.
If I convert a volume to APFS, can I later revert to HFS+?
You can use Disk Utility to erase an APFS-formatted volume and reformat as HFS+. However, your data will not be preserved when you reformat the volume as HFS+.
Rotational hard drives and fusion drives were supported in Developer Preview 1, but support was removed in Developer Preview 2 ... The verbiage on this one was ambiguous. ... FYI -- in beta 13 (Sept 1) -- HDD-only Macs cannot be converted to APFS.
There is no harm in leaving your drive HFS+