This isn't the kind of impression we want you to have while using QuickBooks Online (QBO), @Beancounter. But don't worry, we hear your voice and take notes on your feedback.

At this time, QuickBooks will follow the address entered on the estimate when converting it to an invoice. That's why the old customer's address still shows.

I understand that this behavior gives you difficulties. For now, I encourage submitting feedback directly to our product engineers. This way, they'll review your suggestion and add it to the future update. To submit feedback, you can go to the Gear icon and select Feedback. Then, enter your suggestion and click Submit.

You may want to view all your sales transactions in the future. You can go to the Sales menu and select All Sales. This will gives you a great view of the status of sales transactions, open invoices, and paid invoices. 

We're always here to listen if you have concerns regarding other features. You're welcome to post again or leave a reply below.