My understanding is that you can make this change without penalty, since your flight is before Dec. 31st. Given the chaos, though, you might leave the phone lines right now for people who are trying to deal with rebooking flights canceled over the past few days, and instead see if you can rebook online through MMB. Otherwise, wait until Monday or so to try calling--stranded travelers everywhere will thank you. Oh, and no BA rep can rebook you via a forum on FT, though that would be convenient!