Welcome to Flyertalk dedrater, and welcome in particular to the BA forum. Happy New Year to you and I hope you are resolved to play an active part in this thread.

Did you try the App to make the change? It can be made to work with Expedia tickets. My feeling from reading this is that the DUS agent - who is only on a limited contract with BA - wasn't going to try too hard but it shouldn't be too problematic for him to make the change in FLY (I know how to do it and I'm just a passenger!). What is definitely true is that the GDS rules are not clear about same day changes - though any experienced agent may know about it - also if you made an earlier change to the ticket there is a well known feature where stuff drops off the ticket - there will be a more technical term but never mind - which means that the SDC component is all but invisible to the agent. That latter problem seems unlikely since I imagine the telephone agent would have said something like that. But thanks for sharing that experience, it may help others. More SDC information here: