You harvest by the trichomes, yes if you have some plants ready take them now and let the others mature. Even if only part of a plant is ready you can trim off the buds but leave the stock and as much of the plant as you can to reduce stress and let the rest finish. It does slow the rest finishing but it will finish and be just as good as the others. You just need to figure how much longer it will take and if you need to start back up the nutrients again then flush or just keep using water. If it's going to be weeks still then go back to nutrients for a few more day's then flush again. It's all about the cycle you are going for so give it a try.