We present evidence of reversible optical control of the third-order molecular hyperpolarizability (gamma) of nonlinear optical (NLO) azo chromophore. We show that optically-induced molecular shape change of the NLO dye from the trans to the cis form, by means of photoisomerization which occurs within the picosecond time scale, breaks gamma down rapidly. The anharmonic movement of the electronic cloud of the NLO dye in strong optical field is "blown out" upon optical exitation of the azo chromophores. We show that gamma recovers its initial value upon thermal back-isomerization of the dye to the trans form. This change in gamma can be optically-cycled many times, leading to a novel all-optical light modulation phenomenon. The light polarization and molecular reorientation do not influence this all-optical switching of gamma. We develop a theoretical model that considers a molecular density resulting from an intensity-dependent balance between two molecular species with different molecular third-order hyperpolarizabilities imbedded in a transparent medium. We derive analytical solutions, and we study the effect of the parameters involved in this all-optical process, including the irradiating light intensity and the change in magnitude and sign of gamma. The theory explains the experimental findings and allows a physical insight into this optical control of third-order molecular hyperpolarizability of Light sensitive nonlinear optical isomers.