I don't have an explanation as to what happened, but I've had a similar situation arise with my reservation on 2465 (DFW/LAX) this Saturday 1/22. Ticket confirmation shows 11B, but when talking with an agent yesterday regarding an upgrade request snafu (different story), she confirmed my seat on 2465 as 13B, which IMHO is far less desirable, as it is directly in front of the lav. Her explanation was that there was a "medical emergency" that required my being reassigned. In a subsequent conversation with another agent, however, I was told that the seat change was made on January 17 because of a "change of equipment". Not quite sure what's going on, and my experience might not be related to yours, but it's an interesting coincidence.