I'm not familliar with an "L" fare class for CO--I believe and L in the fare basis code refers to the fare being non-refundable, but it shouldn't appear as the first letter. ARe you sure you're looking at the fare class not the fare basis code?

As long as your OP number is associated with your reservation, you don't need to do anything to sign up for EUA--to check on your chances, you can use ITN or Galileo (there are numerous threads on this board you can search for that give URLs and instructions for these site).

Without having flown on specific legs, it's hard to say what upgrade (yup, you just sit in first class (F)instead of coach (Y)) chances are--as a silver, you get beat out by golds and plats, so it depends how many of them are on your flights. This varies by route, time of day, and day of the week.