Yes, you must do a factory reset to downgrade to 8.1.6.0. I tried a dirty flash and the result was a bootloop. After wipe data everything was OK.
Note: first make a full backup locally; restoring from MiCloud will restore only apps, but not apps data.
Full backup for your app and their data, system settings, contacts, SMSs, notes made with built-in phone app. This is for restoring your personal data easily.