As far as I’m aware it doesn’t downgrade to the free version - you either have to pay to remain using the upgraded trial version or uninstall it. When uninstalling it you should get the option to go to the avast free version.
Personally I feel this is extremely cumbersome and awkward process and doesn’t take into account that many users may not have the expertise or inclination to indulge in this process.
Not user friendly IMHO.
It should simply revert to Free version automatically like many other software programs.