I would not count on the upgrade. I had an FCM of <$250 OW w/ lie flats on that route, and people pull the trigger with it being a 6.5 hour flight. With your flight being so far away, the cabin will fill up and there are plenty of DM/PM that will be ahead of you, even on a Saturday. Assume you will be in Y and book based on equipment. If JFK-EU is 767, book the flight through JFK. If its a 757, I would say you will be more comfortable on the SEA-EU flight which will likely be 767 or 330.