You will need to make use of eCart to make these changes. The first thing to do would be to update the server behaviors that are on the confirm and pp_confirm page. You will need to update the PayPal payments pro server behaviors by double clicking on them in the server behaviors tab. When you are editing them you will have the option to set the PaymentAction. You can also do this directly in the code. The problem with editing the server behaviors is that they make use of custom code, this is why we do not recommend updating them.