You can change the Tax Type on a Sales Order in the Active state and Finalized state if not yet Invoiced/Paid. Click on Edit Order > Change the Tax Type > click on Save. If the Order is Finalized and Invoiced/Paid or Fulfilled, you can no longer change the tax type on the order. In the case of Finalized orders that are invoiced or paid or Fulfilled orders, as a work around, most TradeGecko users void the order and re-create the order with the correct tax type.