This hasn't always been the case, but I'm finding it to be the new norm across the board. Even though you're holding a confirmed seat on SIN-LAX, the system can't reprice changes unless the entire journey is available (as stargold wrote.) Two years ago I was able to replace segments without a problem, but recently I've gotten a lot of push back from SQ on this. Back in July it took me two weeks of calls/emails back and forth with SQ to get them to add an AKL-SIN to my existing SIN-JFK F ticket.

...it's impossible to find open F award inventory on your existing flight when SQ only makes one seat available per segment.