Underground hydrogen storage is a potential long-duration energy storage option for a low-carbon economy. While research into the technical feasibility of hydrogen storage in various geologic formations is ongoing, existing underground gas storage (UGS) facilities are appealing candidates because of their demonstrated ability to store and deliver gas. We estimate that transitioning U.S. UGS facilities from natural gas to pure hydrogen storage would reduce their collective working-gas energy by 75%, from 1,282 TWh to 327 TWh. However, withdrawals from most (73%) UGS facilities could be increased to maintain current energy demands with a 20% hydrogen-natural gas blend. Hydrogen demand projections for the U.S. suggest that hundreds of new underground hydrogen storage facilities may be needed by 2050. Storing pure hydrogen or 20-60% hydrogen blends in UGS facilities can sufficiently buffer this demand demonstrating that partial transitions of UGS infrastructure to hydrogen storage could substantially reduce the need for new facilities.