Sorry, but I want to address this: "in my account quickbook desktop"

It's not "in" anything.

Do you use Word or Excel? These are programs. Each of these programs allow you to use data files. Word is used to manage document-type data files. Excel is used to manage numeric data having formula and functions for doing math.

You can get a Word data file from anywhere. You use your Word Program to manage that file.

You can get an Excel data file from anywhere. You use your Excel Program to manage that file.

Your Client has a QB data file that happens to be on your computer. If it was on my computer, I would open and manage it using my QB program. Since you have their data file, you use your QB program to open and manage it using your QB program. If your client has their own file, they would also have the QB program. They could then even use my QB file, if I sent it to them.

The Word documents, Excel spreadsheets, and QB files are not "in" anything. They are the same as a paper folder of data, but on the computer. You are asking to migrate or transition your Desktop Client's file to using a QB Online subscription? And there is no going back and forth, and the QBO tools are different than desktop.

Here is the community article for your QB needs and the migration topic:

https://quickbooks.intuit.com/learn-support/en-ca/import-or-export-data-files/migrating-from-quickbo...

Make sure you thoroughly understand the instructions.