make the switch like cnd411 said all at once and my personal preference is to extend the dark time, because the plants need their rest and thc production takes place in the dark...you would also be erring toward finish. I always feltlike it shocked the plants more to leave the light on foor 24 hrs. after having them so long in 12/12...
one last note: you are almost there, even if the shock produces a few hermie seeds,
it's not the end of the world. better to have them warm at night, if that is an issue...imho.lw