About reversible single-crystal-to-single-crystal transfor-mation is used to dramatically change the relaxation behavior of a single-ion magnet. The coordination geometry of the DyIII site of a [Zn-Dy-Zn] complex changes from pentagonal-bipyramid (quasi-D5h) to octahedron (quasi-Oh), inducing an energy barrier change from 305 cm-1 (439 K) to a negligible value, respectively. Ab initio calculations reveal that the ideal D5h-DyIII is of perfect axiality with a substantial energy barrier in accordance to the experimental result.