"Rolling" a cheaper item to a more expensive item

My company is a distribution company that has a unique policy.  If we sell item X with a unit price, and it turns out that we don't have X, we will upscale (not change the price) the item to the next highest item - in other words, a 4 pack of soda will actually go out the door as a six pack.  There isnt any customers who have complained.

How do I get GP 10 to roll the sale up from X to Y?  i can't find anything.