I believe I've seen this discussed before, but in skimming and searching I can't find it.

My situation is that my wife and I have award seats booked for Ho Chi Minh City(J)-SIN(F)-NRT(F)-LAX for next year. I see award availability online for Hanoi-SIN on the same day and in the same class (J) as our current SGN-SIN award flight. I'd like for us to change our departure so that we fly from HAN instead of from SGN. But the SQ phone agent tells me that she can't make the change. She says that even though the HAN-SIN leg is available, she can't make the change as part of the larger ticket.

A few questions:

1. Is she correct? (I kind of assume, so but want to check.)
2. Any way around this restriction?
3. Could I waitlist us for the HAN-SIN flight?
4. What's the basis of the restriction? She said that the award seats are only allotted to certain overall itineraries, but I wonder if it's because the rest of the itinerary (SIN-NRT-LAX) is no longer available for new award bookings.

Thanks for any help.