First of all, download (if you don't already have)
DB Commander 2000 Pro.
Then go to C:\Documents and Settings\user\My Documents\NBA LIVE 2004\saves\(whatever your dynasty save number is). Then open players.dbf.
If you want to trade (for example) Dwyane Wade for Kobe Bryant. First, go to Kobe Bryant, and go under the column TEAM (self-explanatory). The number should be 11 (which is the Lakers team number). Just change that to 13 (Miami Heat team number). Then, do the same to Dwyane Wade (change from 13 to 11).
Secondly, go to the column ROSTERPOS (right by TEAM; also self-explanatory). Kobe's roster position should be 3, so there is no need to change it. But if there are two players on the same roster with the same ROSTERPOS number, then if you view the roster in the dynasty, there will be no one in that slot, and if you try to play a game that way, it will crash (this has happened to me before).
Anyway, I hope this guide helped. It was merely an example, I don't mean you should do this trade.
If you want to know more, look at the
players.dbf guide.