  We investigate experimentally a Bose Einstein condensate placed in a 1D
optical lattice whose phase is modulated at a frequency large compared to all
characteristic frequencies. As a result, the depth of the periodic potential is
renormalized by a Bessel function which only depends on the amplitude of
modulation, a prediction that we have checked quantitatively using a careful
calibration scheme. This renormalization provides an interesting tool to
engineer in time optical lattices. For instance, we have used it to perform
simultaneously a sudden $\pi$-phase shift (without phase residual errors)
combined with a change of lattice depth, and to study the subsequent
out-of-equilibrium dynamics.
