Let me help you get this sorted out, charmk.

QuickBooks has a 3 or 4-years cycle for updating. It seems the gap of your QuickBooks data file is a bit long, you may consider trying the interval jump. This way, we can make sure your file data is compatible with the current version.

Let's download the QuickBooks Desktop trial version 2012, then restore the file from 2012 > 2016 and then 2020. Then, create a backup copy as well for 2012, 2016, and 2020. 

Right after, install a trial version of QBDT 2016 and restore the backup file, then create a new backup. Once done, you can now restore the new backup file to your QBDT 2020.

You may also consider checking out this article about the company file compatibility with QuickBooks Desktop when upgrading for your reference.

I'm also adding the link on how to Move your QuickBooks Desktop file to QuickBooks Online for the detailed guide.

Please let me know how it goes. I want to make sure everything is taken care of for you.