Hey bro ..... During the first 2 weeks after your change your light cycle to 12/12 the plants will stretch a lot. You want to keep your nutes still veg nutes to support this explosion in growth. Also, you want to keep the light spectrum that of vegging (so still MH) also to support the growth and reduce the stretch a bit. When your girls stop stretching too much or at least slow down and you notice more and more pistils are forming, that's when you switch to HPS and change your nutes to bloom!
I hope this helps and happy growin