Remove 50% of your foliage. Not growth tops just big light obstructing leaves that are preventing light from getting to the lower half of the plant. The idea is to thin out the plant so it has very good air flow within the canopy, and so that light can get down and put the lower half of the plant in the game.
Let the plant recover over the next week, then flip to 12/12 light schedule and begin week 1 of flowering nutrient mixture and feed.