One possible way to do this is to add a magnet to the encoder disk inside the wheel assembly. The robot wouls see twice as many tach puldes which could fool it into running at half speed.
If you use SCI you really need a wireless connection (or a mind control if that is still available and if it works with the 500 series). You can make the robot drive around and even respond to stimuli.
But you can't change the existing firmware to change the speed.
If the robot sees a constant IR bump due to the index card it may cause an error.
Reds x 3, Dirt Dog, Disco (now a parts bot), Create, Scooba 350, and Security Dawg