Changes in roll centres probably aren't very common, as a lot of people don't fully understand what a change in roll centre means. Basically, to change the roll centre means to raise, or add or remove spacers underneath the suspension arms and upper links. Obviously, a high roll centre would be where the suspension arms and upper links are raised on the car, and a low roll centre would usually be without any spacers whatsoever.

A higher roll centre will normally make the car more responsive, and maybe even a little bit "twitchy" and some people may find this way hard to drive, but in essence, it is normally the quickest way to go. However, if the surface is low traction then it can often be beneficial to use a lower roll centre generate that little bit more grip and make the car easier to drive.

In this section I will also include the effects of kick-up, anti-dive and anti-squat. Now these words sound very technical, but all they refer to is whether there is a difference between the heights of the front and rear mounting blocks for the suspension arms.

Kick-up is where on the front of the car, the front pivot block for the suspension arms is higher than the rear pivot block. This will improve front end response and can be used for bumpy tracks.
Anti-dive is where on the front of the car, the rear pivot block for the suspension arms is higher than the front pivot block. This should be used for smooth surfaces but may make the front end of the car unstable, or understeer slightly.
Anti-squat is where on the rear of the car, the front pivot block for the suspension arms is higher than the rear pivot block. This will help give the rear end more traction and grip, but could affect the absorption of bumps.

Note: Roll centre changes may not be available on all makes and models of car.