A good time to switch the lighting is when you start to see pre-flowers. It's a sure sign of maturity. Other things to consider are height and space. Your gals will probably double, even triple in size during flowering, so consider that as a guide when to flower. The 8-9 weeks is how long it will take your mature plant to flower. You could switch to HPS now. Your plants will like the light, just make sure to keep them at least 12" away from the bulb! Peace!