For many cats, the slightest change in what they're used to can create stress. So if you're thinking about changing their brand or type of litter, plan ahead and decide the best way to do it based on your particular cat.
Method #1. Gradual Transition. When a cat enters the litter box, she expects to encounter the same scent (or lack of) and texture on her paws every time. If she steps in and is confronted with a foreign scent or texture, it could potentially create enough confusion for her to feel as if another location would be a better option for elimination. Unfortunately, her location choice might not be to your liking.
If you have to change brands or types of litter, add just a little of the new kind into the current litter at a time. Each day, increase the amount of the new litter so that the transition lasts about four or five days. To maintain an appropriate litter level, just start removing some of the current litter in the box.
If you're switching from a traditional clay litter to a clumping formula, keep in mind that the new litter won't perform as expected during the transition. READ MORE;
http://pets.yahoo.com/blog/catbehavior/2881/switching-litter-take-it-slow/