The plants probably are tipping away from the light. They went from the flouros to the hps and they were not prepared for the increase in light intensity. If you ever have to support young plants pipe cleaners are great for this as the are easy to work with and have fuzzy stuff to prevent damage to the plant. Next time you move plants from flouros to a HID be aware of this phenomena and keep the light high enough to prevent it (normally 2-3 feet)