The points will show up in your account over the weekend after the flight that qualifies you Silver. Once there (check on the web site) you can use them. IF you have a flight that you booked before you were Silver, you can now go back and change your seat assignments to the better seats, too. When you check in for your first post-Silver flight, make sure your boarding pass reflects your status, for early boarding. Most of all this happens without a glitch!

Like everyone has said... Call EXACTLY 24 hours before your flight to request your upgrade, for best chance of getting it. I had great success on almost all of my attempts when I was Silver. Good luck!