you can turn it back to 16/8 no problem. it will take a little while longer to get it back to veg stage but i dont think that long since it hasnt been under 12/12 for too long. what type of plant is it? indica or sativa? indica you normaly start the flowering at a 1-1.5 of height. i would only re-veg it if it is skimpy and not have a lot of branches.