If the leaves are a healthy green you could wait another week or more, but I don't care much about them being put into soil with slow release fertilizers (which I believe you were saying, but I might have been wrong.) A quality soil without nutrients already in it is preferable. That way you feed your plants the right things at the right times and you can adjust if needed.
You could have started out your seedlings under your 400-watt using your MH bulb, hopefully a 6500K one, so you could switch it anytime you feel like it. As long as you always leave enough space/height between your light and the tops of your plants so you do not create a heat issue your plants will love the extra light.
Use the HPS for flowering, that is what it does best, flowers, and MH is better for vegetative growth (especially if the bulb is a 6500K bulb or at least as close to that as you can get).