A polarised Raman study of YBa₂Cu₃O_{7- delta} single crystals revealed significant changes of light scattering anisotropy with reducing oxygen content. Strongly (y,y) and (x,x) polarised scattering from free carriers and its coupling with the phonon system (Fano interference) are characteristic for the orthorhombic crystal ( delta approximately=0). In contrast, in the tetragonal crystal ( delta approximately=0.8) enhanced continuum scattering and asymmetry lines were observed only in the tetragonal (z,z) polarisation. These findings are attributed to the reduction of free carriers in the xy CuO₂ planes due to chains-to-planes charge redistribution initiated by the decreased oxygen concentration.