The possibility of a significant Fermi-surface deformation as a result of valence crossover in mixed-valence systems is investigated theoretically. The Anderson lattice model on a square lattice with an additional local repulsion between c and f electrons is used to set up a system that shows a sudden valence crossover upon making small changes in the parameters. Analyzing the model using the dynamical mean-field theory demonstrates a significant change of the Fermi surface across the valence crossover. Thus, the structure of the irreducible susceptibility in the momentum-frequency space changes completely, which results in a significant change of effective interactions among quasiparticles. It also smears out high-energy Hubbard satellite bands in the weakly correlated regime, which provides a direct experimental test for identifying a sudden valence crossover.