Do I have to remove the old pads when replacing them with new ones?Updated 2 months ago

Removing the old pads is recommended for best results. There are several reasons for this, and the major ones are:

1. New pads may not stick well if the older pads are rough with holes or claw marks, and they might especially have a lot of trouble staying on if the old pads are dirty or dusty. 
2. If the older pads are falling apart with big bare patches, these low spots will still exist even with the new pads covering them, leading to uneven footing for your cat. 
3. You will likely have to replace the pads more than once through the life of the wheel, too, and the more pads you stack on top of each other, the thicker and heavier the wheel will get. Pads near the bottom of the pile may lose their ability to grip, eventually resulting in big clumps of pads coming loose from the interior of the wheel. 

Completely removing the old pads and replacing them with new ones will provide the cleanest look and the best longevity. If you're having trouble removing the pads, heating them up with a hair dryer will loosen the adhesive and allow you to pull them free. If you're dealing with worn pads that are fragmenting into little pieces when you try to pull them off the plastic, lay some duct tape down over the foam fragments, heat the tape up with the hair dryer, and then peel the tape up—the foam bits underneath will come up with it.