In the install page for river https://doc.e.foundation/devices/river/install I see
It is important that you know that downgrading Smartphones on OS versions greater than the /e/OS OS version you plan to install, can cause instability or at worst brick some devices. We recommend that you check your current OS version, before attempting the installation.
So this “generic” advice highlights only the hazard of downgrade. For this 2019 device please check carefully what you see as the latest stock firmware available and be sure to upgrade to that.
Now you have the chance to install any /e/OS Android version at or in advance of the latest manufacturer firmware.
- As an aside, you might check within your stock Android phone somewhere like Settings > About phone > Android version. Now look for Vendor Security Patch level, it will be a date – the date of the Motorola software revision on the phone.