NO! Each ballast is designed to operate a certain wattage bulb type. If you are using a 250-watt metal halide, you cannot replace it with a 400-watt metal halide without changing to the appropriate ballast. Likewise, you cannot remove a 250-watt metal halide lamp and replace it with a 250-watt high-pressure sodium lamp without changing the ballast. If you do this, the bulb will either not light or greatly reduce the life of both the light bulb and ballast. A gross mismatch could even cause electrical shock or fire.