I didn't miss the context of this being about the unused return half of a non-refundable itinerary, just telling you what I've been told and what I've found to be the practice during my travel experiences.

I suppose there's a very subtle point to what the agent told you (although the agent could just be flat out wrong, too.... ) If you originally bought a deeply discounted ticket that booked into the L fare bucket, then later try to travel on it, and if the flight you want also has L fare bucket availability, then maybe this really would work as the agent described. But if it doesn't have L fare bucket availability, then you'd have to up-fare to the cheapest available fare bucket just as I described. Even if L fare bucket is available, my description would be just as accurate if the published fare is still the same as when you originally bought the round trip ticket -- the "up-fare" would be $0.00. Conversely, if the published fare has dropped, then I expect that the fare drop could be applied to reduce the $150 change fee. I think the only situation where the agent's description and my description would deviate in net results is if there is an L fare bucket seat available, but the published fare has gone up. By the agent's description, you'd pay exactly $150. By mine, you'd pay $150 plus the difference in published fare.