Hello Peter_stark_seve,
Thank your for posting to the community!
If you are changing your VAT number your will need to edit this in the VAT Center often displayed as Taxes on the left hand side of your Quickbooks.
You can click Edit VAT and select Edit Settings. You will see an option to change your VAT registration Number here. This will then feed to your invoices when you create a VAT invoice (An invoice that includes VAT).
Hope this helps.
Thank you.