Hi Ben,
Here at QuickBooks we strive to do everything we can to save you time with your bookkeeping. That's why we've created handy features in the Online program that allow you to do things like convert Estimates into Invoices.
However, this process doesn't convert the other way around (i.e. Invoice into Estimate). I can see how that would be helpful, especially when you're completing similar projects on a recurring basis for your customers. I'd be happy to pass along your suggestion to the software development team for review. When they introduce new updates, there's often new features that were initially suggested by users like you!
In the meantime, here's an article on how to convert an Estimate to an Invoice in case you need it: How to convert an estimate to an invoice.
Let me know if you have questions.
Cheers!
