Not sure what flights you're on... but I plugged in your dates, priced out IAD-SEA-NRT-BKK-NRT-SEA-IAD for $747.50 USD including taxes. Fare basis VWSP7 on the outbound and VXSP7 on the inbound. It's upgradeable using SWUs and it's good for Q miles (no where does it state not good for miles - make sure you ask the agent about this to confirm).

So looks like you might be able to get a voucher for the differences.