Hello @kathycookllc,
Yes, you can work with your companies in each version of QuickBooks. Let me share a few things you need to prepare before the conversion of one of your 5 companies.
To start with, you'll have to install the latest version of QuickBooks Desktop and make sure your company file is not too big for the export process. To learn more about the steps, here an article you can read for reference: Move your QuickBooks Desktop file to QuickBooks Online.
Once the conversion is completed, consider pulling up the same report in each version of QuickBooks and compare your data. This way, you can check if your books do match or not since the conversion process is expected to create a copy of your company file.
In addition, here's an article you can read to learn more about the steps you need to after the conversion process: Post conversion checks for getting started with QuickBooks Online.
Lastly, I've also included this helpful article for the steps when you need to start inviting other users into your QuickBooks Online account: Add, delete, or change user access.
If there's anything else that I can help you with, please let me know in the comments. I'll be here to lend a hand.
