Thank you for visiting the Community, @Ash Gascoigne. I can share some information about changing the vat on multiple invoices.
I can see how convenient and practical it would be if you're able to generate vat with invoices. However, this option is currently unavailable in QuickBooks Online. You can only change it one at a time.
I suggest reaching out to your accountant for a faster way to change VAT calculations. This way, we can ensure that your filings and books are accurate.
For reference, you can check this article to learn how to record tax payments in QuickBooks Online. To manage and troubleshoot them, you can click here.
Let me know if you need further help with your invoices in the reply section below. Take care and have a great rest of the day!