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Let me address your concern about changing the format of your estimate and invoice when sending.
The way to have this feature available to you is to create or edit a custom form style for the Estimate and Invoice.
After performing the steps above, ensure to select the template you've customized when creating an invoice or estimate by clicking the Customize button at the transaction page.
To learn more about how you can personalize your sales forms, visit this link: Customize invoices, estimates, and sales receipts in QuickBooks Online.
If you'd also want to import your customized template, take a look at this article: Import custom form styles for invoices or estimates.
For your future task, you may find this great resource helpful: Convert an estimate into an invoice in QuickBooks Online.
Please let me know if you have other questions. I'd be glad to help you further.