Creating a button to copy transactions in a new form type

I am trying to give my staff the ability to return items that we have purchased from our vendors through a RGA type of set-up. Our vendors require that we create a purchase order, to be a bill and then credit the bill  to complete this transaction.
I figure that the best way to do this is to create a custom Purchase Order form that is labeled "RGA FORM".
When a return is required, the user would simply locate the original purchase order and copy it, change the form to the "RGA FORM" and remove the good items, leaving only the items to be returned.