That is a healthy growth rate 8" in 2 days
Plants could be stretching between internodes cause there getting too much yellow light(
from your hps) and not enough blue light
(metal halide), or maybe your plants are crowded too closely together and are receiving too little light. You should change lights from HPS to metal halide, which would provide more blue light and reduce that stretching.
its one of the 2 or both.
so make sure spacing is fine between plants & put that mh light in & it'll be fine.
then as your into 2 weeks since you flicked switch to 12/12 ,leave mh in & replace with hps when stops stretching as it'll soon be budding up,if not allready doing so.....if is budding up i would just leave hps in as it'll stop stretching & concentrate on budding up.
that has been your problem , church strain is a cross of indica/sativa(flowers in 9-10 weeks) & should not grow high wide & handsome so quickly under a 600 watt light,especially with light 8" away.
all the best M.
ps;