Hi, @capnomo. Let me share with you some additional information about upgrading QuickBooks Desktop (QBDT) and company files.

Once you've upgraded your file from QBDT 2018 to QBDT 2020, you won't be able to open the backup file from QBDT 2020 in the original version of QuickBooks (2018) after the process. However, you can always use the old backup files before the update suggested by Fiat Lux - ASIA. Then, restore it in QBDT 2018.

If you have missing data, you can update them manually or use the WebConnect process for bank entries. 

For more information, while upgrading your QBDT files, you can check out this article: Upgrade your company files after you switch to a new version of QuickBooks Desktop.

Aside from upgrading your file, please ensure to update QuickBooks so you have the latest software release and fixes.

If you have any other questions while restoring or handling your company files, let me know by adding a comment below. I'm more than happy to help. Have a good one!