I'm not sure I understand the reasons for trying to attempt this? APFS is going to be far better for you than HFS+..
If APFS is work as expected. That's good, however, it is not.
1) Finder unable to correctly calculate and report empty space.
2) Slow boot when TRIM activated
3) Finder micro freeze
4) Bootcamp apps in Windows unable to selected APFS as next boot partition
5) Few APFS related KP (seems fixed in the later High Sierra)
6) No RAID / Fusion support
7) Causing About This Mac -> Storage very slow on getting the space analysis.
If HFS+ is garbage, then APFS is far worst than garbage at this moment. And it's amazing that this garbage HFS+ works reliably on my Mac in the last 10 years without issue. But the fantastic APFS not even work as expect for 1 day. All the above happened on my own Mac. I tried APFS for 3 months, multiple OS update, still not completely fixed.