In one sense, I kind of hope they would NOT do that. How does the GA know that the people in the other reservation have given you the authority/permission to change their reservation (even if you have the same last name)? That is treading on VERY thin ice.
I made the reservation, I'm on the reservation. How would this be different than voluntarily changing the reservation? I can do that alone online or on the phone or at the airport after paying the change fees and difference in fare.
You're right, the GA does not know for sure that the people traveling with me want to change their plans. But that is true no matter the circumstances of the change. Delta did not know for sure when I made the reservation in the first place that the people traveling with me want to go. Correct me if I'm wrong.