So your current routing is HKG-SEA-JFK? You used to be able to SDC from a regular F routing to a D1 routing, but they have (officially) removed this option. You might find an agent willing to help you out here.
One other thing to consider, though, is that if you are on a GUC, you will probably be at the top of the upgrade list, so they might let you SDC to the flight in Y and clear the normal upgrade list.
And yes, you can only request it after all international segments on your itinerary are complete. If you have a return to HGK on this ticket it won't work.
If OP clears the GUC on DL38, they'd be on Delta One ticket. Should be no issue rebooking as far as the F to D1 rule.
OP, yes can only be done upon arrival to SEA. If you've already cleared then it should be no issue to move. However prepare for HUACA due to incompetent agents. It's not their fault, but Delta's for cheaping out on training.