My experience is that you get more luck at the outstations than at LHR. I have been successfully put on stand-by at check-in and then given a new boarding card in the lounge for an earlier flight from JFK twice, and once at YYZ they did it at check-in. This happened twice on I-class and once on a W+UUA=J, so clearly not the ticket type that allowed it. Being a GCH and terribly British and apologetic about even asking probably did help