I can share with you some details on what happens when you download or convert customer data in QuickBooks Desktop (QBDT), Lisa.

It will not damage your customer's online files if you convert their data to QBDT. However, there are certain exceptions. 

Take a look at the following items:

• The length of display names is restricted to 31 characters.
• Both the full name and the company name are limited to 41 characters.
• Each line of the Billing and Shipping Address is restricted to 41 characters.
• The length of an e-mail address is limited to 81 characters.
• Customers who have been deleted are changed to inactive customers.
• Default terms for sub-customers are adjusted to reflect the default terms for their parents.
• Due to PCI compliance, customer credit card information does not export.
• Customer notes, preferred delivery method, mobile number, pager number, and is taxable status are not exported.

Refer to this article for further information on how your data converts from QBO to QBDT, aside from your clients' information: Limitations when you convert from QuickBooks Online to QuickBooks Desktop. 

To ensure your books match and the information is exported correctly, I suggest running the Profit and Loss report in QuickBooks Online. Then, compare it to the same report in QBDT on accrual basis. For more information, see Step 5 in this article: Export and convert your QuickBooks Online data to QuickBooks Desktop.

Post a reply below if you have any further queries regarding downloading your clients' information or anything else. I'm always willing to assist. Have a fantastic day!