Would agree that using the SWU for a flight to NRT would be better. I used a SWU to upgrade a NRT-IAH flight a week ago, though it only cleared one week before departure. Had a harder time using my SWU for EWR-PEK and PVG-EWR flights earlier this year (they did not clear). So based on my (very) small dataset, it would seem that NRT has a slightly better chance than other Asian destinations, but I'm not sure if these SWU stats are representative of the February timeframe.