If you upgraded your Mac to macOS Mojave and have decided to downgrade for one reason or another, the procedure is usually pretty straightforward. Fusion volumes, however, introduce a complication. Upon upgrading to Mojave, a Fusion volume will be converted from HFS+ to APFS. If you want to downgrade to High Sierra (or any earlier OS), you must reformat that Fusion volume as HFS+. Because APFS Fusion volumes are not handled gracefully by High Sierra, however, the procedure is a bit tedious.