Hi there GrantTheGoalie,

When creating the invoice from an estimate, there isn't the option to modify the existing line from the estimate to add quantity, but what you can do is add another line to the invoice with the additional hours. I can see that being able to modify the existing line would be a feature that you'd like to see to help you manage your invoices. I recommend leaving feedback in the system by clicking on the Gear icon, then Feedback. The comments made in that section are taken into consideration by our developers when designing new features for QuickBooks Online.

If you have any other questions, feel free to reach out here.