Cabinet Transformations® Complete Coating Kit can be applied over previously painted cabinets as long as the paint is in good condition and adhered well to the cabinets. There is no need for any stripping or sanding, you would simply use the deglosser in the kit to prepare the surface, following the directions provided in the kit.
Please note that if you are going from a very light colour to a very dark colour (or vice versa), this may require additional coats or material.