Hello there, @Cherokee99.

You can link an existing invoice to a project in QuickBooks. You just have to make sure that the invoice is not already linked to other transactions like estimates and billable expenses.

To do that, you can follow the six steps given by my colleague ZackE so you'll be able to add an existing invoice to a project in QuickBooks Online.

Please know that it's not recommended to add invoices linked to more than one transaction because moving an existing invoice to a project will remove any linked transactions such as payments, credits, billable time, and attachments. What you can do is add also the linked transaction to the project and reconnect them to the invoice.

You can come back to this thread if you have further questions. I’ll be more than happy to help you out.