[Ernst]

Gear shifting is done with either pneumatics or servos. If you have a pneumatic system on your robot, plumbing in two small cylinders will not be difficult. I have found pneumatic shifters to be quicker and more reliable than servo powered ones.

To add shifting you will have to replace your gearbox and potentially much of your drive-train of you are modifying an existing robot. Adjusting and readjusting chains or belts with idlers is no simple task, especially if you haven't designed the robot in CAD.