Upgrading Fare Class (to select seat) on JAL Multi-City
 

I'm trying to book a multi-city trip (SEA->SGN, SGN->TOY, TOY->SGN) and the experience for booking on the JAL site for multi-city leaves a lot to be desired. I eventually found an itinerary that works but some of the legs are an O class and won't let me pick seats. Does anyone have advice for how to see what the price difference would be to bump to a class that will let you pick seats?

Hi there. First, confirm what booking classes allow seat selections (here), and then find the fares on ITA matrix as mentioned above.
Note that the O class bookings you saw likely are on either American or Alaska metal.
Tokyo is TYO (Haneda/Narita), not TOY, FYI.
Also, JAL doesnt fly to Seattle. (routes), so, you need to be clear what routing you are looking to use to get to a JAL gateway on the west coast.
As to what booking code lets you book seats, you can use the info in the link I provided and check on JAL website what fare codes are aligned to the fare codes that allow seat selection.
Just from a random search, you would be looking at a mixed ticket on several carriers to make your trip work.
Just as a by the way, do you have to fly JAL?