If you are Gold and not that far from the Airports, or arriving on transit flight, then it may be worth doing it at check-in or the lounge - you never know, they may do a deal and let you go all the way for the same miles! Happened to me once, i offered on the CHC-SYD sector, I was booked in Y and SYD-DXB booked in J, they bumped me to F all the way from CHC-DXB via SYD and BKK.