The magnetoresistance of a two-dimensional electron system, subjected to a one- dimensional lateral potential modulation is studied for various potential amplitudes. The interplay between Landau level splitting and modulation broadening is shown to generate a transition from even to odd filling factors for the Shubnikov�de Haas resistance minima. The effect provides a tool to determine the potential amplitude. Calculations within a one-particle model agree with this evidence of the van Hove� like structure of the Landau bands.