Switching from 24/0 to 18/6 isn't a problem at all. I leave the lights on 24/0 while rooting clones, and then switch to 18/6 (or 12/12) with no problems. For plants in veg, I prefer 18/6 because it's simulates an outdoor environment better. I've also heard that roots don't develop quite as quickly under 24/0, but that may just be a myth.