Yeah there is a similar posting just about every week with this problem. You are stuck and won't be able to upgrade until both segments have upgrade space. As UnitedSkies said, next time use the multi city option on United.com when booking and do DEN-SEA and SEA-NRT as separate segments. NEVER book a direct flight with a stop as 1 segment.

The other negative you're going to experience is that UA will give you award miles for a nonstop DEN-NRT instead of DEN-SEA-NRT. In this instance the difference is negligible (6 miles). But there are plenty of routes where the difference is much bigger, such as SEA-IAD-FRA (flight 916, segments flown separately you earn 6,386 miles) but if you book it direct you only earn the nonstop SEA-FRA (5,108 miles).