A new scheme to control dispersive bistability is proposed for etalons with Kerr nonlinearity. Analysis with an equation of motion for slowly varying amplitudes shows that the bistable states can be switched by an optical input consisting of a bias and pulse component. The switching time is as short as the pulse duration and is compatible with a high Q-factor of the etalon. Appropriate selection of the amplitude and phase of the pulse is essential. High-Q microcavities formed in AlGaAs-based photonic crystals are prospective candidates for implementing the prescription