If you change the configuration (seating configuration or class of service Economy, Business etc) the aircraft goes into an extra check. Also if the aircraft maintainance level falls ... i.e. 93%, you can start an extra check to bring the level back up to 100%. The extra checks depending on the amount to be fixed usually includes A or B checks.
You can find it under my Aircraft, and look at maintance view, and you will see the next scheduled check and aircraft condition. You can then go to the maintance icon and start extra check if you wish to keep the fleet condition topped up.