Your root growth will slow down when your plants start to flower. Unless it's an emergency of some sort, I wouldn't transplant this late into flowering. You don't want to do anything that will shock the plant at this stage.

Your plant will also stop stretching soon. But I have had some sativa strains that will grow out of control. I've had to bend the plant and tie it down to keep it under height. LST is your friend at this stage of the game.