Repainting walls that have a knock down texture.

So my husband paints but he mostly does industrial painting and prevailing wage jobs on new structures but my parents asked him if he could paint thier house and we really need the money. So they have knock down texture on the walls and they are a med to light grey which they want painted a very light gray color. Do they need to be sanded first so that the paint sticks better? Will sanding it ruin the texture? Can someone please help me understand the correct steps that need to be done to get the wall painted right?