Can I link a paid invoice to an existing estimate if the invoice was not created and linked from the estimate originally?

Hi shannonlogan444-,

Welcome to Community!  QuickBooks Online is a dynamic program for completing your daily bookkeeping tasks.  I can see the importance of tracking invoices and estimates.  I'd be glad to provide some insight here!

QuickBooks Online makes it easy for you to convert an estimate into an invoice.  This will update the estimate status to 'closed'.  You'll be able to open it to view the linked invoice.  However, you won't have the option to convert an invoice to an estimate, or to add the invoice to an existing estimate.  If you feel this would be a beneficial feature, you can submit a suggestion to our Development Team, for future updates.

Please don't hesitate to reach out with other questions.  We'd be happy to help!