Backwards Compatability 2020 to 2010

Welcome to the Community forum, @Simon Kirkman.

A license of QuickBooks Pro 2020 is only applicable to install on QuickBooks Pro 2020. You won't be able to open a company file to a lower version of QuickBooks if you've already upgraded the file to 2020.

To learn more about the company file compatibility, you can refer to this article: Company file compatibility with QuickBooks Desktop when upgrading.

If you have a backup copy of your previous version (not converted file), you can open it if you decide to use your old QuickBooks version. 

I’m only a post away if you need anything else. Wishing you the best!