Tricky,as you'd need to change the source. However, you could always run a bit of SQL after you've done the refund, like this:
Code:
UPDATE Orders_ShoppingCart
SET OrderedProductSKU='<your sku in here>'
WHERE OrderNumber=<your order number here>
eg
UPDATE Orders_ShoppingCart
SET OrderedProductSKU='ABC123'
WHERE OrderNumber=102011
As always, I recommend trying this on a test environment first to see what the ramifications of doing this is.
What if your customer orders 3 different items on one order and you are refunding on 2 of them? How will you handle 2 different SKUs? Make 2 ad hoc refunds? Or combine the 2 SKUs into 1 hybrid SKU?