High fares matter a lot. If you are on a B or Y fare I would say you have a 50% chance of being upgraded to first BOS/SFO as a 1K. I was 2/2 on my trip last week (Sunday 6:00am departure, Monday 6pm return). I wouldn't count on mid-week upgrades as a 1P. There are lots of tricks though, buying your tickets late means that you can look at the load factors and choose the planes that have seats left.