Changing light cycles?

I am presently growing Blue Mystic in a small grow box. They only have 2ft to grow so i will change their light cucle after about 3 weeks but was curious if I should work the light down to 12 hrs or just go straight from 24 to 12/12?
I don't want to screw these up so the help will be much appreciated.

straight to 12/12 bro

sweet, thanks.