  A crossover between different power-law relaxation behaviors of many-body
periodically driven integrable systems has come to light in recent years. We
demonstrate using integrable quantum systems, that similar kinds of dynamical
transitions may also occur in the relaxation of such systems following a sudden
quench. Particularly, we observe two distinct power-law relaxation behaviors
following a sudden quench in the integrable XY model, depending upon whether
the quenched Hamiltonian lies in the commensurate or the incommensurate phase.
The relaxation behavior for quenches at and near the boundary line, called the
disorder line (DL), separating these phases is also characterized. The
relaxation at the DL shows a new scaling exponent previously unexplored. The
transitions occur through a crossover from the commensurate/incommensurate
scaling behavior to the DL scaling behavior. The crossover time diverges like a
power law as the parameters of the final quenched Hamiltonian approach the DL.
The transitions are also observed to be robust under weak integrability
breaking perturbations but disappear following strongly chaotic quenches.
