An efficient method of changing steps in the numerical integration of astrodynomical equations: LIU Cheng-zhi, Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030;Changchun Artificial Satellites Observing Station, National Astronomical Observatories, Chinese Academy of Sciences, Changhun 130117 & CUI Dou-xing, Changchun Artificial Satellites Observing Station, National Astronomical Observatories, Chinese Academy of Sciences, Changhun 130117