Pics help.
Soil I assume? What are your temps?
Seedlings or clones? More information would be helpful, but generally once the plants have had a week or two under CFL's and have started to show some real growth, they are ready.
But are you going to veg them under the 600 HPS? Or flower them right away?
If you are going to veg them more, they are gonna stretch under 600 watts HPS...
But if you don't veg them more, they may not yield a lot if they are still small heading into flower.
How many plants/how big is your area/what's your goal for yield?