Hi there, accounts.
For now, QuickBooks Online can't copy an invoice to a new credit note. However, we can link a credit note to an invoice.
It would be great if you can also submit a feedback for this one through the Gear icon then select Feedback. We have a product release every month and most of the enhancements come from the customer's feedback. As we strive to become the most user-friendly accounting software, your voice really matters.
If you have questions, please let me know. I'll be happy to lend a hand.