If you don´t already got the tallest riser then you can easily change them. The only "problem" that can occur is the hole for the fittings (so they won´t turn). If that´s the case you just drill another hole or cut off the pins of the fittings. With some tape underneath they fit tight enough again and won´t turn.
You saw the pictures of the different riser?
http://www.gs-500.info/index.php?tit...3.B6cke_der_GS
Just compare with it yours.
The tallest riser were attached from 1996-2000.