Assuming they are from seeds not cuttings then if you switch to 12/12 too early then they will just continue to veg until the first point that they are ready to flower. They will lose some light and that could mean they will come out of veg marginally smaller than if you absolutely nailed the timing but it is not really significant if you are there or thereabouts anyway. Assuming you are vegging them under a light powerful enough to flower them (which it looks like) immature plants will easily take on enough light during a 12 hour period to veg effectively. I have grown from seed under 12/12 before and they just flower at the point that they are ready, it's not as efficient for the reasons given but before autos came about I did it so I could have vegging and flowering plants in a single chamber. They'll roughly double in height/diameter so if you have the space then you might as well use it... even accounting for the extra ttime spent, having bigger plants at the end of veg is an easy way to get a better yield if you have the room.