Why not just click "change itinerary", then select new flights for the one leg, and see if it shows a net increase or decrease in points? You don't have to go through with the change if it's a point increase.
I "changed" some award tickets to the same flights last night. I thought the fare on the outbound dropped by a few points but couldn't be sure. Then it showed me a 900 point refund per ticket before I confirmed the "change".