Changing shipping method on fulfillments

Hi, everyone,

One of our particularly large customers creates a problematic scenario for us.  When we ship to this customer, we ship UPS most of the time.  However, we can't be sure how we're going to ship until we fulfill the order, then send that info to the customer who then decides how to ship them.  As such, we default all the orders to UPS. 

Sometimes the customer changes the shipping to FedEx.  Since our salespeople don't have the ability to edit fulfillments, I have to manually change the fulfillment every time this happens, and then change the SO to match.  I would like this process to happen automatically when the SO is changed by a salesperson, but I can't find a good way of doing it.  I tried scripting the change, but when the script runs, it gives the error