I suspect this, or some variant such as due to quick processing or technical difficulties with SDC, commonly results in the waiver of a fee without the passenger ever knowing. For example, a SDC change on a flight that has been upgraded with a RUC often results in a change of class from upgrade class to a "C" class booking. I had it happen twice recently on the same itinerary. In both cases the agent initially told me there was no space to move to the new flight, even though there was FC availability. When I gently pushed back on them, they made the change and I ended up with what looked like and credited as a FC fare. I'd be willing to bet there was a waiver in each of those situations. Interestingly, on that same itinerary, I had a 3rd change (trip was a CF) and on that change the agent did it correctly and I stayed in whatever the RUC upgrade class is (OY or OU?)

Makes you wonder if that recent letter regarding waivers from DAL that was posted to OMAAT and linked in this forum was real. Lot of people thought it fake, but I'm not so sure it wasn't just a poorly worded letter by a non-well supervised CSR.