Buyers Workbench

We are using version 12.1.3 and use the requisition account generator to create distributions based on the employee expense account and the purchasing category selected on the requisition line. Using the standard functionality a non-catalog requisition is raised in i-procurement, the category and item description is selected and the account generator creates a distribution. When the requisition is approved it is available to the buyer in the Professional Buyers Workcentre. The buyer uses the functionality to change the non-catalog item  for a catalog item and this has a different purchasing category to the one originally selected. As such the distribution should be