Based on Pendragon's citation it sounds like APFS will be mandatory (automatic) only if you upgrade to High Sierra (10.13). Otherwise you keep what you have.
Reminds me of the HFS -> HFS+ and the APS -> GUID transitions I've been through in the past. I hope the tools I'm used to using (like ditto) continue to be compatible.
I also recall trying to install OS X on an APS formatted drive on an intel mac. After that, the OS X installer refused to run because "your computer can't start from this drive". That was back in what, 2010 or so? I think even to this day you can boot an intel from APS. I kept my service drives formatted this way so one drive could boot intel or ppc (since ppc definitely could NOT boot off guid) and it was always a minor headache to set up a new one.
