If you get upgraded at the 48-hour mark, the system will assign you a seat in F. You can/should bring up your reservation shortly after the passage of the 48-hour window to see if you got upgraded and are satisfied with the seat assignment. If you want a different seat you can bring up the "change seats" subroutine and see what else is available.

Sometimes you will get a "good news!" phone message and/or an email congratulating you on having achieved this noble benefit. But my experience is that these notices don't always come about and you are better off taking matters in your own hands.

If you don't get upgraded at 48 hours, you might get upgraded at the 24-hour mark. When you do the OLCI, on the screen that gives you the option of changing to an earlier flight or a later flight (if such options are available), there will also be a choice to "upgrade to first class" that shows up if such option is available for you. Of course, you should accept this option!