Bostonflyer - BOS/SEA isn't one of the easier routes to upgrade on. "U" class of service has to be open at the clearance window for a complimentary upgrade. This is the same class used by mileage upgrades and Gold Guest Upgrades - so there's a risk it won't always be available at 72/48/24 hours prior. In a perfect world, we then clear at the gate prior to departure unless it opens up and clear in our appropriate order.