The option to change a transaction category in the register is currently not possible. You'll want to delete and manually add the transactions to your register. Ensure that you copy the details of the transactions before deleting it and use the expense as the category. Let me show you how.
In your QuickBooks Online (QBO) account:
Go to the Accounting menu.
Select Chart of Accounts (COA).
Found the account register you want to view and click View register.
Click the transaction you want to delete and copy the details.
Then select Expense from the Add check drop-down menu.
Fill in the details and click Save.
Once done, open the transactions you want to delete and click Delete.
I'd also suggest getting in touch with your accountant in making these changes. This way, we'll ensure the accuracy of your books after recording this. Also, you can ask your accountant if you want more insights on how to better record these changes.