12/12 to 18/6 or 24/0

if i started a few plants on 12/12 can i change it to 18/6 or 24/0 anytime??

Pretty much. Not sure why'd you start a plant on 12/12, but yes, you can add light.

well i just put it in with my plant thats flowering so im gonna move that plant in a few days.
just making sure i can add light
Thanks =P