After the VAT conversion with the option ‘Convert VAT percentage’, a new VAT code with 19% is now present for your history. The 21% VAT code has been linked to various masterdata (for instance, an account).
While creating an order, invoice or purchase invoice, the VAT code of 21% will be used by default and in order to still use the 19% to enter, you should adjust the VAT code in the order, invoice or purchase invoice.
This may, however, not be possible. A possible cause of this is that you have linked a VAT code in the account maintenance of the relevant account, where you indicated by using a lock that this VAT code may not be deviated from. In that case, it is not possible to change the VAT code.
To solve this, you should open the lock and thus unlock the VAT code. You can achieve this by clicking the lock icon behind the ‘VAT’ field in the maintenance of the debtor/creditor on the ‘Financial’ tab.
The VAT code cannot be changed either in a sales invoice that comes from an order. The order determines the invoice here, so in order to change this invoices after all, you can delete the invoice. The corresponding order will then be opened again. You can then change the VAT code and generate the invoice again via [Order, Entries, Print/Process].