I had an invoice with 14 entries on it that I had to void and a year later recreate. I thought it would be easiest to make a copy in QB of the original invoice and then just go down and re-enter the hours (I copied off the original invoice that I found in my email). The problem now is that the estimate shows those hours as unbilled and that is not great - I don't think my method was a good one, I would caution anyone who bills off estimates not to do this.
I also like the idea of seeing some sort of warning that an invoice is connected to an estimate before you can void it.
Apparently, the current warning we can see says that there are one or more payments connected to the invoice, and not about having a connection to an estimate. For this reason, I recommend sending your suggestion by clicking on the Gear icon and selecting Feedback. That way, our engineers can consider adding one in future updates.
On the other hand, you can check out the Training menu above if you want to see some video tutorials for QBO.
I'm just a comment away if you have questions about QuickBooks. Take care and have a good one!