How do I apply the new sales template to existing sales invoices which are set up on the default template?

Thank you for choosing QuickBooks Online (QBO) as part of your business, James. I'll help you with updating the template of your existing sales invoices.

You have to modify the existing invoices manually to use the new template. 

Here's how:

1. Open the existing sales transaction.
2. Go to the Customise tab.
3. Select the new template from the options.
4. Click on Save to record the changes.
5. Repeat the same steps for other invoices.

For more tips about personalising your form templates in QBO, you can open this article: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Aside from your invoices, you can also get more hints about handling your sales and other customer entries through this link: Topics about your company's income and customers.

If you have any other questions about updating your transactions' template, let me know by adding a comment below. I'm more than happy to help. Wishing you a good one!