Originally Posted by amanuensis

For domestic flights, silvers can change seats to any available Economy Comfort seats at no charge once the 24-hour window is hit. But unlike First-class Upgrades this will not be done for you automatically -- you have to manually change the seat when you check in online. So of course the early bird gets the worm.

If you don't want to wait and risk no domestic Economy Comfort seats being available at the 24-hour window, you can book a Y, B, or M fare and be able to select any available Economy comfort seat at the time of booking at no extra charge. Or you can pay to upgrade to an Economy Comfort seat at any time AFTER booking (like, one minute after booking) at a 50% discount.

For international flights, silvers only get the Economy Comfort seat for free on Y and B fares. Or if you happen to get an operational upgrade, which is unlikely -- and you won't know until you are getting ready to board. The only likely way that a silver will be able to sit in Economy Comfort on an international flight is if s/he pays the upgrade fee after booking. For international flights, the silver discount is only 25%.