Greetings, @CCIOffice. 

Welcome to the Community. I'd be more than happy to provide you with some more information about linking existing invoices to an estimate. 

I can see how beneficial this could be for your business. However, at this time, you'll either need to do convert the estimate to an invoice going forward or delete and recreate the invoices. 

I recommend submitting feedback to our Product Developers so they can consider this in the near future. Check out our Blog site to keep up with what's new in the system. 

I hope this helps answer your question. Don't hesitate to come back if you have any other concerns. Best wishes!