Use the Hps from start to finish. If you wanted you could just add the 400 MH to the scheme of things.If you took them out from under a 600 and replaced the lamp with a 400 at the same height.You will for sure have stretching.What controls stretching is how close you are able to get your lamp to the tops of the plants.The strain has alot to do with stretch,too.Hope this helped ya some!

Ga Grown!