Project upgrade to 2019, error, how to roll back to the version before upgrade

My project was upgraded from 2018.3 to 2019.3, and an error occurred after the upgrade. How to rollback to the version before the upgrade? The version before the upgrade is 2018.3, thank you

 

you can't.
always make backups before upgrading (or better, use version control).
especially to an alpha version...

you can try to make a new project, then manually copy your assets (keep the .meta files) but there is no guarantee it will work.

 

……

 

Can I return my project to any version of 2018? thank you

 

No, you can't go back to any version prior the one you are using, when making the upgrade there is a warning that you should make a backup.

 

So, you upgraded your project but you didn't keep a copy of it before? I guess you learnt your lesson.

Now, what about fixing the error?

 

It is possible to downgrade but might break some of your stuff. Just open the project in the older version and see what happens. You will probably have to edit your package manifest and maybe delete your library folder and projectsettings but it should work.

 

That can sometimes work, and just depends on the file changes Unity made between Unity versions.

 

There is a big scene serialization change in 2018.3 -> 2019.1. I screwed up before when I thought my plugin that was quite neutral could go back and forth between versions safely, but I forgot I have a demo scene. 2017.1 <-> 2018.3 is relatively more possible to up-downgrade, as long as I touch the scene only in 2017.1. Try looking in your source control if there is a diff in your .unity files or not. And when changing version backwards always delete your Library and Packages.

Try use a branch feature in version control when you are going to upgrade and keep your master in the past. If you upgraded successfully, don't merge yet until that version come out of beta. When you find a blocker after working quite a lot you could go back to master and work on other things that is not conflicting with what you already worked on in the future version, then later you could merge.