The magnetoresistance of the molecular beam epitaxy grown GaAs/Al0.3Ga0.7As superlattice with Si-doped barriers has been measured in tilted magnetic fields in the as-grown state and after brief illumination by a red-light diode at low temperature, T approximate to 0.3 K. A remarkable illumination-induced modification of magnetoresistance curves has been observed, which indicates a significant change of the superlattice Fermi surface topology. Analysis of magnetoresistance data in terms of the tight-binding model reveals that not only electron concentration and mobility have been increased by illumination, but also that the coupling among two-dimensional electron layers in neighboring quantum wells has been reduced.