Though charge transport is sensitive to subtle changes in the packing motifs of molecular semiconductors, research addressing how intermolecular packing influences electrical properties has largely been carried out on single-crystals, as opposed to the more technologically relevant thin-film transistors (TFTs). Here, independent and reversible access to the monoclinic and triclinic crystal structures of a core-chlorinated naphthalene tetracarboxylic diimide (NTCDI-1) is demonstrated in polycrystalline thin films via post-deposition annealing. Time-resolved measurements of these transitions via UV-visible spectroscopy and grazing-incidence X-ray diffraction indicate that the polymorphic transformations follow second-order Avrami kinetics, suggestive of 2D growth after initial nucleation. Thin-film transistors comprising triclinic NTCDI-1 consistently outperform those comprising its monoclinic counterpart. This behavior contrasts that of single-crystal transistors in which devices comprising monoclinic crystals are consistently superior to devices with triclinic crystals. This difference is attributed to more uniform in-plane charge transport in polycrystalline thin films having the triclinic compared to the monoclinic polymorph. As the mobility of TFTs is a reflection of ensemble-average charge transport across crystalline grains having different molecular orientations, this study suggests that among different polymorphs of a particular molecular semiconductor, those with smaller in-plane anisotropy are more beneficial for efficient lateral charge transport in polycrystalline devices.