wow thats long for vegging, those plants must be pretty big. I had a small sativa clone a while back and switched it over from 18/6 to 12/12 right away. IDK about your 24 hours, i guess you might want to lower it to 18/6 for a few days and then go to 12/12 and see how it goes. Weed is a weed so it can take a hell of a lot of stress, and as long as your plants are ready to flower you shouldn't have any problems going directly from 24 hours to 12/12.