Apple - 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.

Note: this was true before the announcement of High Sierra, June '17

You cannot use APFS as a boot volume for Sierra, it's not yet supported.

APFS is still in development & will be capable of being used as a boot volume when it's finally released.

From Apple File System Guide - FAQ ...

Can I boot macOS Sierra from an APFS-formatted hard disk?

No. macOS Sierra supports Apple File System for data volumes only. You cannot boot macOS Sierra from a APFS-formatted volume.

How do I upgrade to Apple File System?

Apple will offer nondestructive in-place upgrades from HFS+ to APFS for all boot volumes when Apple File System ships in 2017. Tools will be available to convert external volumes from HFS+ to APFS format.