Hello there, MJBs.
You're correct. The sub-customers you created will remain unchanged if you downgrade your QuickBooks Online Plus to Essentials.
For future reference, read this to get a copy of your lists and reports through an Excel file. You can refer to this article for the details on how to do so: Export reports, lists, and more.
There are also articles that tell you more about handling subscriptions: QuickBooks Online Account Management.
I'm always here if you still have questions or other concerns. Let me know in the Reply section below. Have a good one!
