Can't really tell at this point. Just looks like another set of leaves growing.
Seems a little early to switch to 12/12. They would have kept vegging for quite awhile and grew a lot. You could have topped them if you wanted to keep them shorter. They would have bushed out more and produced more colas.
There is also another option called "Low-Stress Training (LST)" you can do a search on these forums to find out more info, but basically you just slowly bend the plant by tying it down with a rope or string. Not only will this allow you to control the height, but it also allows the sun to hit the sides and other parts of the plant that wouldn't normally receive large amounts of direct sunlight, which will result in the production of new bud sites all over the plant.
Other than that, they are looking very healthy. You should invest in some flowering/bloom nutrients with a high Phosphate and Potash ratio since you are starting flowering. Let us kno if you are near any hydro stores or just hardware stores like lowes or homedepot, so we can recommend nutrients.
Good luck!