I am running into an issue with a multi-city ticket. It goes City A - B - C - B - A. I am booked in D class from A to B and B to C. I am attempting to change B to C to another city that meets the fare rules of the ticket, but there is no longer D availability on the A to B portion. There is D class available on the new flight from B to new city and the agent confirmed it does meet the fare rules, but because there are not two ADDITIONAL seats available on A to B they can’t make the change.

Does this seem right? I have two D seats on that flight, why do they need two additional seats on the leg I’m not changing to make the change?