Surely the distance selling regulations come into play and you have 24hrs to say that you don't want the upgrade. I've done this when I had booked an F trip to BOS and then discovered that night that there was going to be someone on my flight back in Club. So I called BA the next day and asked to downgrade the return flight so that I could sit next to them. This was done with no fuss and the difference refunded so if it matters to you call back ASAP and get it refunded.