This seems like a scam. I paid an extra £15 for a Plus fare to get free SDC on the return leg of a day trip, try to change it in MMB this morning but get an error (no surprise - BA IT...), so have to ring up, after 15 mins on hold the agent tells me that he can only change my return leg if he offloads me from the outbound flight I'm about to board, so I instead have to fly the outbound and then call them back to try again. I doubt I'll have a spare half hour today to hang around on hold to BAEC, so now I'm probably stuck with my original flight.

£15 well spent...