We uncover a previously unreported solid-solid phase transition in tetrachloro-$m$-xylene (TCMX), a hexasubstituted benzene derivative exhibiting orientational disorder. Differential scanning calorimetry and powder x-ray diffraction reveal a weak but reproducible phase transition near $437\text{K}$, corresponding to a continuous, second-order symmetry change from monoclinic $P{2}_1/n$ (phase II) to orthorhombic $Pnnm$ (phase I). Despite this structural reorganization, quasielastic neutron scattering demonstrates that molecular dynamics remain governed by discrete ${60}^{\circ }$ in-plane reorientational jumps consistent with the pseudosixfold molecular symmetry. The activation energy is unaffected, while only slight variations in relaxation times and effective rotational radii are detected. Molecular dynamics simulations reproduce the transition and clarify its microscopic origin. In phase II, weak orientational correlations between adjacent molecular columns are present, but these constraints vanish in phase I, restoring higher symmetry without altering the underlying reorientational mechanism. This study establishes TCMX as a rare example of a disorder-disorder transition in which molecular dynamics are preserved while collective orientational correlations reorganize. More broadly, our results highlight how subtle symmetry changes govern emergent behavior in disordered crystals, advancing the understanding of plastic phases and phase transitions in complex condensed matter systems.

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• Crystal structure
• Crystal symmetry
• Order parameters
• Phase transitions
• Second order phase transitions
• Solid-solid transformations
• Transition temperature
• Differential scanning calorimetry
• Molecular dynamics
• Quasi elastic neutron scattering
• X-ray diffraction