Exploring　the　transformation/interconversion　pathways　of　catalytic　active　metal　species　(single　atoms,　clusters,　nanoparticles)　on　a　support　is　crucial　for　the　fabrication　of　high　efficiency　catalysts,　the　investigation　of　how　catalysts　are　deactivated,　and　the　regeneration　of　spent　catalysts.　Sintering　and　redispersion　represent　the　two　main　transformation　modes　for　metal　active　components　in　heterogeneous　catalysts.　Herein,　we　established　a　novel　solid-state　atomic　replacement　transformation　for　metal　catalysts,　through　which　metal　atoms　exchanged　between　single　atoms　and　nanoalloys　to　form　a　new　set　of　nanoalloys　and　single　atoms.　Specifically,　we　found　that　the　Ni　of　the　PtNi　nanoalloy　and　the　Zn　of　the　ZIF-8-derived　Zn1　on　nitrogen-doped　carbon　(Zn1-CN)　experienced　metal　interchange　to　produce　PtZn　nanocrystals　and　Ni　single　atoms　(Ni1-CN)　at　high　temperature.　The　elemental　migration　and　chemical　bond　evolution　during　the　atomic　replacement　displayed　a　Ni　and　Zn　mutual　migration　feature.　Density　functional　theory　calculations　revealed　that　the　atomic　replacement　was　realized　by　endothermically　stretching　Zn　from　the　CN　support　into　the　nanoalloy　and　exothermically　trapping　Ni　with　defects　on　the　CN　support.　Owing　to　the　synergistic　effect　of　the　PtZn　nanocrystal　and　Ni1-CN,　the　obtained　(PtZn)n/Ni1-CN　multisite　catalyst　showed　a　lower　energy　barrier　of　CO2　protonation　and　CO　desorption　than　that　of　the　reference　catalysts　in　the　CO2　reduction　reaction　(CO2RR),　resulting　in　a　much　enhanced　CO2RR　catalytic　performance.　This　unique　atomic　replacement　transformation　was　also　applicable　to　other　metal　alloys　such　as　PtPd.