I have not heard of a 600w working in a 1000w, but that does not mean its not out there. You could actually get 2 600w HPS lights for your grow room, now that will mean more heat, but you will be around the 180k lums range and if you get a digital ballast it will help with the electric bill. What you really should do is figure out what your total electric bill will be while in the veg stage and then you will have an idea of what type of light you should use for flowering. The ideal thing to do with veg/flower lighting is to be 1/3 your light. So if your veg'n under a 400w you would want to have 1200w for flower. So, if you already have a 400w, you could simply go to a store and get some cfl's and then you wont have to worry about the 1000w. I'm not really sure on the difference detail on the difference between 400w and 1000w for flowering as far as bud performance, but it will be just fine for what i think you are trying to get.