This is the main menu that is presented at the start of the game. While there are default rosters and pitchers they can be changed at this point for either team. Once the games starts if a player is taken out of the lineup or a pitcher is replaced they can be come back into the game, just as in real baseball.
Here you can see that Yogi Berra has been selected to replace Mickey Cochrane for the 7th place in the batter order. I will post a short video to show how easily this is done by first selecting #7 which is the number of the player to be replaced then #14 for the replacement player. The program then asks to confirm before making the choice. If the changes are being made before the game has started then they can be changed back. If it is after the game starts the player removed is now in active and will be shown in a different color on the bench.
See the video here : Roster Change Video