Teams can change their 23-man active roster anytime during the regular season. There is no limit to the number of active roster changes a team can make over the course of the season. Active batters placed on the bench can not be reinserted into a team's active roster for 3 days. Pitcher's fall under the 6-day minimum contract rule. This means that any pitcher placed into your active roster can not be benched for 6 days.
Players can change positions in the field without incurring any time on the bench.
For example, if you wanted to move Chipper Jones from third base to corner infielder on your active roster, you could without losing any games played. Teams may request to acquire a player on waivers. If more than one team requests the same player, the player will be awarded to the team that has waited the longest period of time since its last waivers acquisition. All players not claimed off waivers become free agents. Free agents are any players not listed on another team's roster.