Hello!
This is a great question and I'm glad to hear that you're considering both subscriptions for your client's products. I'll go over this to make sure your client successfully makes the switch from QuickBooks Desktop to QuickBooks Online.
QuickBooks Desktop and QuickBooks Online have unique subscriptions, which means one cannot be transferred to the other product. When you're migrating the data from Desktop to Online, it's important to make sure the Desktop subscription is cancelled once the migration is complete. That way you or your client won't be double-billed when only using the one product.
You're free to make the switch whenever you'd like. To learn exactly how this will affect the QuickBooks Desktop subscription, I recommend touching base with the Desktop support team for more details. You can see how to get in touch here: Intuit QuickBooks Desktop software support policies
Here are some additional details about converting data from QuickBooks Desktop to QuickBooks Online for your reference.
Best of luck!