Originally Posted by TennisNoob

Can never bump which is the most hated rule in my opinion of both JL and NH. The agent you spoke with last that confirmed you is wrong and won't be able to make the change. He's for sure wrong! You can only change fare classes from J to Y and still pay the J price. You cannot go from Y to J and pay the extra miles even if applicable. But for your case since you already paid J you're only going to get the Y seat you purchased.

If you knew the long haul was in Y you shouldn't have done J from ICN-HKG because you're paying the J rate from JFK-ICN.

That being said yes the only way is to cancel the whole itinerary and re-do it. I would check on UA.com to see if seats are available for your original itinerary first. If so than you have no risk of cancelling and rebooking.

If you see no seats and you cancel than you're screwed. Just because they originally released X amount of seats doesn't mean you will get your seat back even if there's another side waitlisted or not.