If done carefully and properly, there should be no problem at all and the plant will not even know anything happened. The main thing is to be sure to get all of the soil surrounding the roots out of the current pot at once. This can be accomplished by letting the soild dry a bit so that it sort of hardens, but is not dry to the point of it being just powdery dirt.Or you can do it by making sure the soil is pretty moist, but not actually wet. If it's in a plastic pot, simply cut down the side of the pot and the plant, soil, and rootball should all come out as one. Make sure your new pot is right there with fresh soil on the bottom, a few inches should be fine, then slip the old soil with all the roots into the new pot and surround with more fresh soil. I've transplanted literally millions of plants over the years, not all marijuana but a lot of them too, and have never ever had any signs of stress from transplanting. The people who say it can overstress a plant or have bad effects are possibly people who think it's just a matter of ripping a plant out of one pot and dropping it into another. Be careful with it, keep as much of the original soil around the roots as possible, and transplanting should be no big deal whatsoever.