thinks to remember when doing a opgrade

Hi all
What should I remember before opgradeing from wm 2002 to wm 2003. I have tjeked all my drivers if they would work with 2003 and they do. What about the ao5, does it install when opgrading? And can I go back to 2002 if it does not work, and then again why shouldn't it.
Sorry about my spelling.