Thanks for joining this thread, @user 12. I'm here to provide information about accessing two company files in QuickBooks Desktop (QBDT).

Fiat Lux - ASIA is correct that you can open two company files in 2 different versions of QBDT if you're using the QuickBooks Accountant or QBDT Enterprise.

If you have the supported version, here's an article for your reference to open a secondary company: Open and use a secondary company file in QuickBooks Desktop. This article includes some restrictions when opening a secondary company file.

On the other hand, upgrading your company file in QBDT is irreversible. Thus, it isn't possible to downgrade your file. What you can do is utilize a backup copy of your company file from a previous version if you have any and access the file in your older version of QBDT.

Moreover, you can check out this resource to avoid issues and errors when upgrading a QBDT company: Company file compatibility with QuickBooks Desktop when upgrading.

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