Originally Posted by
tboons
Similar experience. SIN to HND. Available online for new award booking but cannot change for the existing award. So called in to make the change and the phone agent said no availability. In the end, I booked the new date with a new set of miles and had to pay 30usd to cancel the old award. Totally rubbish
Your case is slightly different from OPs.
He had confirmed SGN- SIN -JFK booking, wanted to change to PHN - SIN - JFK, could see PHN-SIN available online, ( but not the whole PHN-SIN-JFK journey). As mentioned upthread, married segments rule at play here.
Your is a different case, SIN-HND shows available online, but agent cannot see it. That does happen from time to time, although should not.