By combining experiments and numerical simulations, we investigate the redistribution of quasimomentum in a gas of atoms trapped in an optical lattice when the lattice depth is rapidly reduced. We find that interactions lead to significant momentum redistribution on millisecond time scales, thereby invalidating previous assumptions regarding adiabaticity. Our results indicate band mapping is an inaccurate probe of the equilibrium quasimomentum distributions of interacting bosons in the single-band regime. © 2012 American Physical Society.