The magnetic viscosity, and the reversible and irreversible susceptibilities of a sample of anisotropic sintered Nd-Fe-B have been measured starting from various initial magnetized states. It is shown that the heights of energy barriers preventing the reversal of the magnetization in the grains depend on the initial magnetic state of the material. The change in energy barrier distribution is shown to be accompanied by a change in the nature of the magnetization reversal process, which cannot be described solely in terms of simple model pinning mechanisms.