You've come to the right place,muenstermann.
In QuickBooks Online, you can simply input the estimate number you want to associate on the transaction under the Estimate # section. When recording a new estimate, it'll use the previously entered one. If you wish to change the number, you have to do it manually. You may also consider using a third-party application to help you correct and keep the estimates numbering in order.
Also, I've added these articles on how to change and customize your sales forms in QuickBooks: Customize your invoices, estimates, and sales receipts in QuickBooks Online.
Keep in touch if you have any other questions about QuickBooks. I'd be glad to answer them for you. Have a good one.
