Here is a link for the 2004 Fed Softball Changes.
http://www.nfhs.org/sports/softball_rules_change.htm
Main change is the DP/Flex rule change.
3-1-5 (F.P.) Replaces the Designated Hitter (DH) Rule with the Designated Player/FLEX Rule. The main differences being: 1) The role of the DP (offensive player) is never terminated, it is possible for a team to go from 10 to nine players and back to 10 players any number of times during the game; and 2) The DP (offensive player) and the FLEX (defensive player for whom she is batting) can play defense at the same time.
Ooy Vay!