We report on the observation of a change in the field dependence of the critical current density Jc of DyBa2Cu3O7/(Y0.45Pr0.55)Ba2Cu3O7 superlattices at a characteristic magnetic field H∗s. This change shows up as an abrupt transition when analyzing the current-voltage ( I−V) characteristics in the framework of the vortex glass and collective flux creep theories. This behavior can be traced back to a sudden reduction in the collective pinning energy Uc above H∗s, and may reflect a softening transition of the vortex lines in the vortex solid. H∗s is very similar to the characteristic field H∗l in the vortex liquid phase, which we interpreted to be due to entanglement.