A changing method includes the steps of supplying a carbon fiber material with a surface covered by a thermosetting resin oil, performing a desizing step to remove the thermosetting resin oil from the surface of the carbon fiber material, performing a surfactant coating step to coat a surfactant onto the surface of the carbon fiber material, and performing a sizing step to cover a surface of the surfactant by a thermosetting resin oil, so as to obtain a carbon fiber material with a thermosetting resin oil coated onto the surface of the carbon fiber material.