Yes, you can install shiplap over existing drywall or plaster walls. In fact, this is a common practice that can save time and effort in wall preparation. To do this, make sure your existing wall surface is clean, dry, and free from any imperfections. You may need to remove any protruding nails or screws and patch any holes or cracks. Then, you can attach furring strips directly to the existing wall using appropriate anchors or screws. Once the furring strips are in place, you can proceed with installing your shiplap boards as usual. This method allows you to achieve the shiplap look without the need for extensive wall demolition.