I'll provide some insights about converting an invoice to an estimate in QuickBooks Online.
I can see how this feature can benefit your business, but the current customer workflow in QuickBooks Online doesn't have the option to change an invoice to an estimate; however, we can convert estimates to invoices.
You can also look for and use third-party applications that will help you change an invoice to an estimate. To do this, you can go to your Apps menu or use this link to find one: Apps help QuickBooks run better.