I'm not sure if it's like the current 2003 game but if it is it's fairly simple. Pause the game. There should be a manage bullpen option. You'll then see a list of pitchers. Click the pitcher you want. Then move the cursor so it highlights "bullpen". He'll then be in the bullpen warming up. Then when he's warmed up go to the manage bullpen menu again and click the pitcher who is warming up. Then drag up up the "On mound" and click it. He should then be your new pitcher. As I said, that's how it works in ASB 2003 so I'm not sure how different it is in ASB 2000. Hope it helps.