Sometime in the last year I showed up at the airport (booked in "R" DEN-IAH-EWR-LHR) and, knowing that there was DEN-IAH-LHR availability, simply asked the agent if I could change. No problemo/no cost. Being Plat probably helped.

I just don't remember if it was before or after the $25/$50 SDC rules went into effect.

ETA: It was last November.