My son and I took a last minute trip SEA-ONT a couple of weeks ago. We redeemed 10k miles each for the one way flight on Thursday, checked in Friday and then stopped at a kiosk on the way into the airport in SEA early Saturday morning to reprint boarding passes. Pushed on the upgrade button and got us both seats in F for $50 each. So the story about restrictions on fare classes that get the upgrade option, not sure they are always in force.