Just miss the flight. They say they will reaccomadate you for free, if there's space. I am doing that intentionally tomorrow, definitely shouldn't be more than the $75 same day change
P.s. my reservation wasn't ticketed - (don't know why BA.com let it go through <24hrs before departure). The EC call center was closed, but a reg reservation CSR took my number and got an EC rep to call me back.