Have you checked your ballast to see if it's rated for both. My powerplant ballast runs both MH & HPS without the need to switch or use a conversion bulb & has HPS/MH on a plate on the ballast. There's been a few questions I have seen recently on this & I have previously read on a lights site that any HPS ballast can actually run MH bulbs. There reasoning given was based on the similarity of componets in each ballast with the main difference being the starter required in the HPS ballast. I will see if I can dig up the bookmark however it was almost a year ago when I first read up on HID lights.