On the light intensity when you switch to flower shouldn't be a problem as long as your Temps at the top of the canopy are reasonable. Also it depends if you Veg under that light already, or veg with weak fluorescent lights or if your starting flower with a rooted clone. If your doing the rooted clone and the weak veg light options, then you would want to raise the height of the light and bring it down a couple of inches each day till you get to the optimum distance away from your canopy. Depending if your light is air cooled or just a reflector with also determine your light height. Unless your battling high Temps don't turn down the dimmer switch, just raise the bulb. But say if you vegged with your 600 watt m.h. light your could go straight to the 600 watt hps without worrying about having the harden them off to more intense light.