Converting existing AC transmission lines to DC operation (AC-to-DC line conversion) can increase transmission capacity without constructing additional transmission corridors. However, after conversion, the impact of HVDC systems on overall system reliability is difficult to quantify. Therefore, a hybrid AC/DC system reliability assessment method incorporating AC-to-DC line conversion is proposed. First, conventional bi-pole HVDC (BP-HVDC), redundant bi-pole HVDC (RBP-HVDC), and tri-pole HVDC (TP-HVDC) configurations are analyzed to establish correspondence between subsystem operating states and transmission capacities. Second, a HVDC reliability model is established based on the parameter-matrixed frequency and duration (FD) method. Finally, using the IEEE-RTS79 system as the test case, sequential Monte Carlo simulation is used to sample the states of AC components and HVDC systems. Combined with an optimal load shedding model, system reliability indices are obtained to evaluate the impact of AC-to-DC line conversion on the reliability of hybrid AC/DC systems. The results indicate that the impact of AC-to-DC line conversion on system reliability depends on the original transmission capacity limit of the lines; converting lines that operate close to their capacity limits can improve system reliability and enhance new energy accommodation capability.