A VAT number can only be added to a Native Instruments business account, as opposed to a private account. In this manner you can exclude the VAT (Value Added Taxes) from your purchases. Read more about the differences between a business and a private account in this article.
It is not possible to change the VAT number of a business account once the original VAT number has been set. If you require a different VAT number, please create a new user account registered under a different username / email address.