Everything I am reading indicates the conversion to APFS takes place as part of the High Sierra installation and is NOT OPTIONAL.
Here's recent details (6/19/17) from Ars Technica
Quick Look ...Overall, an interesting preview.
Looking at the second paragraph under APFS, note that the current Beta allows user to
uncheck the APFS box (it's checked by default). Continuing in the follow-up paragraph, Ars Tech says High Sierra can boot from an HFS+ drive.
So unless something changes in the final version, we may be able to determine whether or not to complete the APFS conversion when installing High Sierra.
