Transmission Raman spectroscopy experiments were performed as function of time on iron doped congruent lithium niobate crystals within two –in principle equivalent symmetric- configurations, namely Y(zx)Y and Y(xz)Y according to Raman selection rules. Whereas only the lines corresponding to E[TO] phonons are detected at any time as expected in the Y(zx)Y configuration, the spectra in the Y(xz)Y configuration shows a large evolution with time. Just at the beginning of illumination, the spectrum in the transmission geometry is very similar to the corresponding spectrum in back-scattering, but rapidly new lines appear and increase in intensity with time. After one minute of illumination, the spectrum completely differs from expected E[TO]. These new lines in fact arise from another mode symmetry and are clearly assigned as due to A1[TO] phonons. This activation with time of forbidden A1[TO] Raman lines for this configuration result from photorefractive internal field which causes the polarization conversion from ordinary to extraordinary polarization and is originated by photorefractivity of the crystal.