Hijacking this thread as I am in a similar situation to OP.

In my case, I managed to redeem SIN-HKG-JFK/EWR-HKG-SIN at the old lower return rate of 145k a pop; this is a confirmed and ticketed reservation. If I wanted to change the return sectors so that we would fly out of JFK instead of EWR, could I do that without any extra cost in either fees or miles? I am currently waitlisted on one such flight ex-JFK on a different reservation, and there is also outright availability for redemption on JFK-HKG-SIN one day after my current departure date.