For anyone who’s in a position to test the 15 to 16 upgrade tool, it’s documented in this wiki page:
https://wiki.freepbx.org/display/FOP/Upgrade+to+FreePBX+16
We’ve been testing internally for a while without any issues, so it’s ready now for wider testing. Issues can be reported to https://issues.freepbx.org/ and discussed here in the forum.
WARNING: FreePBX 16 is beta software. We recommend against upgrading production systems at this time.