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.
On a Mac since 1984. Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.
