A few months ago I OLCI'd for an EDI-LCY flight convinced that I would make it. As the day progressed it was clear that I wouldn't and I needed to get on a later flight (I had a fully flexible Business UK ticket).
I rang BA who were able to reverse the check-in process (voluntarily offloading me I suppose) and then change my flight. The person on the phone confirmed that there was no way of doing this online.