Yes, you can switch them back to veg at any time, but there is a time price to be paid....two or 3 week delay while they revert. Just up the light back to 16+on and they will go back to veggin, but they won't grow much while doing this and you may see some single blade or mutant looking leaves for awhile when it does start veggin again. They eventually return to normal again tho. Then grow em out to what you want, and switch them back to 12/12....there is also a slight risk of herming them from all the stress of doing this.