Polymorph conversion of transition metal dichalcogenides (TMDs) offers intriguing material phenomena that can be applied for tuning the intrinsic properties of 2D materials. In general, group VIB TMDs can have thermodynamically stable 2H phases and metastable 1T/T' phases. Herein, we report key principles to apply carbon monoxide (CO)-based gas-solid reactions for a universal polymorph conversion of group VIB TMDs without forming undesirable compounds. We found that the process conditions are strongly dependent on the reaction chemical potential of cations in the TMDs, which can be predicted by thermodynamic calculations, and that polymorphic conversion is triggered by S vacancy (V-S) formation. Furthermore, we conducted DFT calculations for the reaction barriers of V-S formation and S diffusion to reveal the polymorph conversion mechanism of WS2 and compared it with that of MoS2. We believe that phase engineering 2D materials via thermodynamically designed gas-solid reactions could be functionally used to achieve defect-related nanomaterials.