Hi SmokyLungs. I can tell you from experience because I have re-vegged many times either by accident or on purpose lol. The plants that are already in flower will get ruined. They will not continue to add weight, they will get stunted once you switch over to the 18/6 light schedule. Usually the buds get super airy and the whole plant will be like popcorn buds. If you're lucky they won't go hermy. The seeds forming on the currently flowering plants is not what you need to worry about, but rather they can form pollen sacs pretty quickly. They could release the pollen in your room and even if the new plants aren't flowering yet, you will have trace amounts of pollen in your grow room that can remain floating around in the air and then pollinate your other plants once they do start flowering. Even if that's a month or 2 later..
Your 6 week flowering plants, once they are no longer stunted, will start re-vegging and grow like a crazy bush and have irregular growth with strange looking leaves. If you're familiar with the monster cropping technique for clones, that is exactly how the re-veg will look. Stringy, long stretching, and the hormones will be wacked out and cause branching in all directions. Sometimes people like that effect because it creates much more tops, but you will be very aggravated trimming the buds because there will be much more weird leaf matter to cut off than if they flowered the normal way. The current buds on those plants will become airy and sprout new tops from above them. So the current buds will turn into shit, but the new buds should be okay as long as they don't go hermie. But again, you will hate the trimming process most likely. Every strain differs but I've done this a few times! I hope that gives you a better idea of what to expect lol.
