Hi Poul,
There are two ways to obtain what you want:
1. Press the SET-button from the sound arranger. Now you can select a different bass or drum sound for the preset rhythm. Remember that you also can make your own sounds and drum-kit.
2. Copy the preset rhythm to the composer. Now you can edit every detail you want.
Advantages: 1 = quick (and dirty), 2 = far more flexible, but takes a lot more time.
Regards, Ad.