actually I have had the best results with 24/0 with autoflowers, I harvested almost twice the amount of bud from ones i did 12/12, I have never tried 18/6 so youll have to wait for someone esle to answer that. Now as far as switching the light cycle, it is possible but should be done in small intervals. example is each day increase the amount of light by 1-2 hours which ever you feel you can do better, autoflowers should pull through if you go little by little.