Unity > Unity project downgrade, from 2020 to 2019

Unity project downgrade

Target

The project originally developed with the Unity 2020 editor wanted to be opened with the Unity 2019 editor, but the following two errors occurred.

The type or namespace name ‘xxxx’ could not be found (are you missing a using directive or an assembly reference?)

Roughly passed

Anyway, the project has been backed up , so make it hard!

1. Delete the C:\Users\{username}\AppData\Local\Unity\cache folder
2. Delete the Library folder of the project, open the project with a lower version (2019) editor and let Unity automatically regenerate the folder
3. After running the downgraded project, I found that I couldn't see the game characters, only the game background. Compared with the original project, it is found that the Sprites such as Circle are missing (Missing), and then the Package is found to be different. Then install the package that is still missing in the downgraded project in Package Manager , and manually create Sprites such as Circle in Assets and add them to the game character.
4. Run the project, the downgrade is successful! Unknown!

Please advise!