FastPass can be retrieved anytime between 45 minutes and 2 hours OR about 5 minutes after a previous FP ride time (say the Splash Mountain FP was for 10 AM - 11 AM, you could get a new one at around 10:05 AM), whichever is first. According to what I've heard, during less busy times, a FP ticket booth may give you a free extra FP for a different ride at a random time (never seen this yet though).
As far as changing FP, other than adding new rides constantly to it, the changes are minor or for the better. Anything you want to know about FP though can be found at
http://members.aol.com/ajaynejr/fastpass.htm