To exchange an order item with another one, select the action "Change product" from the drop-down :
This will open a popup from where you will be able to select the product to exchange with.
Simply click on "Select" in the "Action" column to select a product :
The price and qty of the initial product will stay after it have been echanged.
The purpose of exchanging a product with another one is to get correct order details during the order preparation process.