Is it possible to convert APFS to HFS+ I'm deciding to install MacOS High Sierra on my Mac this autumn.So if I find APFS not useful can I convert it back to HFS+ via terminal or using Disk utility?
This autumn of 2017 when High Sierra is released, there will be far more concrete details and evidence for you to make an educated decision. I can’t conceive Apple won’t make clear the risks and benefits on consumer, support, professional and technical levels for APFS. My guess is most people should make one backup and then jump. If you have old software that cannot be updated, you might wait three months for any odd bugs to get documented or worked out if you can’t easily recover from a bootable backup or reinstall your apps at the versions you require. 

At worst, you can always wipe and reinstall High Sierra on HFS and then copy files from your backup (whether the backup is HFS or APFS or some other file system)



In July 2017, The migration scripts and functions are still being tested in beta - so today, there are no signs you could revert back from APFS to HFS on the beta macOS. Worse, you shouldn’t even trust the forward conversion yet for non-throwaway system test use. 

Many programs will not work yet on APFS formatted volumes. 


https://bombich.com/blog/2017/07/07/pondering-conversion-from-hfs-apfs


I would recommend keeping all your important data and all your system backups clean and untouched by beta versions of APFS scripts. 

What seems to work well even now in the beta is copying files back from APFS to HFS if I work on a file or photo in beta and want to keep it on my main system. That’s a very different thing than reverting the entire file system structure, though.