I disagree with the gradual change. Change them to 12/12 immediately. When you originaly put your plants into flower, you change them all at once in one day, change it right over to 12/12. I keep the sunrise time the same. I put my plants into a 24 dark cycle of darkness to change them over from veg to flower. I've read it gives them a good shock, and promotes the release of a flowering horomone.