I admire you for taking the time in telling us about your concern, @capnomo.

The process of upgrading your company file in QuickBooks Desktop (QBDT) is irreversible. Thus, it isn't possible to downgrade your file.

As a way around, you can look for a third-party app that lets you revert your 2020 file back to the 2018 file. Simply hover to the App Center in your QBDT or visit our Apps for QuickBooks Desktop page for reference. 

Moving forward, you'll want to create a backup copy of your company file before you upgrade it to a newer version of QBDT. This way, you can still access your file in your older version of QBDT.

Furthermore, I'm attaching this article that you can utilize in case you need to import some data to your QBDT: Import and export data in QuickBooks Desktop.

You can always count on me if you need more help with your company files or with QBDT. Keep me posted by tagging me in your reply. Keep safe and have a good one!