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Hello, @AndyAllan.
Yes, you'll have to undo the reconciliation as suggested by my colleague @MariaSoledadG. When you change the VAT Status of your transactions it may increase or decrease the transaction's total amount.
Here's a screenshot on how it looks like when you change the VAT Status of a reconciled Sales Receipt:


Also, I'd recommend consulting your accountant so you'll be guided accurately in making VAT-related adjustments.
Feel free to click the Reply button if you have other questions. I'm just a few clicks away.
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