Migrate from HFS+ to APFS in-place
As of Jun 2017 according to this link, during macOS High Sierra install, file system can be upgraded from HFS+ to APFS:


  How do I upgrade to Apple File System?
  
  The macOS 10.13 installer offers nondestructive in-place upgrades from
  HFS+ to APFS for bootable volumes. You can use Disk Utility to convert
  external volumes from HFS+ to APFS format.