Originally Posted by
gcashin
I tried doing a SDC out of LHR in May on a P fare, and wasn't successful. There were seats available in J on the flight I wanted at the airport 2 hours before the flight and the flight was not oversold, but there was no P class availability. Both the ticketing agent and concierge in LHR insisted that in order to do a SDC, there needs to be positive space PLUS your original fare class needs to be available. In order to change at the airport when the original fare class isn't available, they insisted I'd have to pay the change fee plus the fare difference for a last-minute D fare. When I asked them to try do do a SDC, I got "computer says no."
So, you may also need to have your original fare class available too, but this isn't clearly published and may not always be the case.
I’ve done an SDC many times out of LHR on P and Z fares. It can be done, but it requires a knowledgeable agent. I had the same response as you from a few agents. The check-in staff don’t usually know how to do it. I started going straight to ticketing and with one exception they were always able to do it. When I inquired about the inconsistency one of the knowledgeable agents said the others are lazy or don’t know how to do it. If you get push back (eg. “computer says no”), tell them to call the help desk. Stand firm. SDC does not require space in the same bucket, only cabin. I’ve done this at least 7 times at LHR before I become SE (now I just call in
)