Unfortunately, clean install is your only option, you can’t downgrade from Pro to Home. Changing the key will not work. You could try the following work around then test your luck:
- Open Registry Editor (WIN + R, type regedit, hit Enter).
- Browse to key HKEY_Local Machine > Software > Microsoft > Windows NT > CurrentVersion.
- Change EditionID to Home (double click EditionID, change value, click OK). In your case it should at the moment show Pro.
- Change ProductName to Windows 10 Home. In your case it should at the moment show Windows 10 Professional.
- Browse to key HKEY_Local Machine > Software > Wow6432Node > Microsoft > Windows NT > CurrentVersion, change the same two values as in steps 3 and 4 above.
- Close the Registry Editor.
- Insert the Windows 10 Home installation media, do an in-place upgrade by launching the setup from desktop (not booting with Windows 10 Home install media).
- When asked, enter your valid Windows 10 Home product key and select what to keep (settings, personal files and apps, only personal files or nothing).
- Windows 10 Home in-place upgrade will be done now.