Ok, I bit the bullet and upgraded to High Sierra. It all went smoothly and quickly. I boot from an external SSD (the internal old school HD has Sierra, rarely used, and Boot Camp, Windows 10, sporadic use). On checking the SSD (Sandisk) remains Mac OS Extended (Journaled). I also have an external HD for Time Machine. Is this satisfactory? Is there any gain for me, far from a power user, to worry about AFPS? How would I go about converting it if there is any great and amazing reason to.