Yes, you should switch to a flower nute schedule that still has "some" Nitrogen in it. By "some" I mean a small amount. It's too early to totally eliminate it IMO. To explain the stage of your plants, one has to move away from "thinking indoor" and realize that in nature, a photo-sensitive plant like cannabis is triggered to flower as day length gets shorter. Once you get past the summer equinox (longest day of the year, usually in June for most) the plant begins to sense the days getting shorter. When it senses this, it begins a hormonal change from vegetative growth to flowering. This is what you are seeing now. Your plants have gone from transition to being firmly in flowering mode. 12/12 is used indoor since all but the full-blooded sativas will flower to maturity under this schedule. The full-blooded sativas I am referring to are Thais, Hazes, and a few African sativas. On those, most will slowly move the light schedule back even further than 12/12 so that the nights are longer in an effort to get them to finish. Sometimes I don't explain things well, so let me know.