In order to evaluate the potential risks associated with retrofitting existing natural gas pipelines for hydrogen transport, an investigation of the disaster evolution mechanisms and risk characteristics associated with natural gas and pure hydrogen transport gas under high-pressure pipeline failure scenarios was conducted. A comprehensive framework was established using FLACS to compare the accident consequences before and after the conversion of natural gas pipelines to hydrogen service, with particular emphasis on leakage dispersion, jet fires, and vapor cloud explosions. The differences in hazard characteristics, including leakage source behavior, flame morphology, thermal radiation, and shock wave overpressure, were systematically analyzed, and key hazard indicators associated with pipeline leakage-induced jet fires were quantified. The results indicate that, under identical pipeline operating conditions and failure scenarios, hydrogen leakage exhibits a substantially wider dispersion range in both the vertical and horizontal directions, with the vertical dispersion height reaching 24.8 m It is approximately 2.58 times that of natural gas, indicating an amplified potential hazard zone following hydrogen conversion. Hydrogen leakage-induced jet fires exhibit substantially higher heat flux intensities than natural gas jet fires, reaching peak values rapidly within a short period and maintaining stable high-intensity radiation, thereby producing a wider thermal radiation hazard zone and increasing the potential for personnel injury and equipment damage. Following hydrogen leakage and subsequent vapor cloud explosion, the propagation velocity of the shock wave and the vertical range of overpressure effects are greater than those associated with natural gas, with the maximum overpressure reaching 2.84 MPa. In contrast, natural gas vapor cloud explosions exhibit more pronounced horizontal propagation effects in the near-ground region surrounding the failure pipeline. These findings provide a reliable data basis for accident consequence assessment, risk zoning, and emergency response planning associated with the conversion of existing natural gas pipelines to hydrogen service.