Good morning
It doesn't matter if you just replace the bulb with an LED or the entire fixture. The power savings is strictly in the LED portion. The LED lights come with the required base to just plug in so we didn't replace any fixtures. We have changed all the bulbs in our class c to LED and it saves a significant amount of power. We went with the White LED because we don't like the yellowish color of the warm white. Also greatly reduces the amount of heat given off by each light. Required when you live in AZ..
Dave