Low-carbon transition of energy systems features more natural gas and hydrogen consumption to replace coal and oil. Planning of natural gas and hydrogen supply systems with multiple supply sources, end-consumers, large infrastructures and spatio-temporal mismatches are challenging tasks. In this study, two long-term, multi-regional and monthly time-scale optimization models for natural gas and hydrogen supply systems are developed respectively. China is taken as a case. The natural gas demand will initially increase and then decrease, leading to a premature decommissioning of infrastructure. A coupling model is established to explore the retrofitting of decommissioned natural gas infrastructure for hydrogen supply. Results show that 44.14 % decommissioned pipelines and 34.17 % decommissioned storage facilities of natural gas supply system can be retrofitted, with an annual increase of 63.98 % and 62.80 % in hydrogen pipelines and storage facilities, and a 9.15 % reduction in transition costs.