Amending VAT against lines when recording an invoice

Is there a way to amend the VAT against individual lines when recording an invoice through POP? I have tried to view the order in the hope that I can change the vat rate at this point but not able to. I am not sure if I need to change a setting to allow this to happen or if it just can't be done. I don't want to have to close the invoice, manually change the vat on the order and then go back to record the invoice every time the VAT is wrong.