Originally Posted by
channa
1Ks can change awards for free.
If the entirety of the ticket is AAA-BBB-CCC as you described, you can cancel the segment you no longer need.
Now if there's more to the story (e.g., this is a return and now you're adding a stopover/open jaw/etc.), it will complicate things.
But if not a 1K, and you checked in for your later flight but didn't fly an earlier segment, will the system already know that you'd skipped that first flight and not let you on? That's an interesting question. No way would I consider doing that on an outbound segment, since the itin would likely get flagged at some point and the rest of the trip killed.
Perhaps for an award ticket there'd be less oversight on such things? It would certainly be a major issue for paid flights, since it's sometimes the case that an MRY-SFO-ORD flight is considerably less-expensive than an SFO-ORD. They wouldn't want people booking the extra segment to save money and not fly it.