We ended up getting a double upgrade when we flew from LHR to JFK a couple of weeks ago. I had originally booked World Traveller using Avios, when it came to OLCI we had been upgraded to WT+, and as we got to the boarding gate, the agent said they had to reissue the tickets and presented us with three seats in CW. Ticket stated "involuntary upgrade due to oversales". This was a Wednesday morning flight, and coincided with the ending of the US school holidays and start of the English/Welsh school holidays which may have been a factor in the overbooking. Not sure if the Silver card helped as well, but we weren't complaining!!