Well i think i may had some misunderstand about nanite documentation at fall back mesh segment before and after several tests i found that if you want to dynamic switch back original LODs in nanite enabled project at runtime, there might be at least one way.
According to the documentation, for a nanite enabled static mesh, we can set Fallback Triangle Percent to 0 and set Minimum LOD to 1 then the engine will use those custom LODs from LOD 1 to LOD n you’ve set. So if you want to keep LODs, just re-import LOD 0~n to LOD 1~n+1, then set bDisallowNanite=true when the LODs are required.
By the way, it will scale up the resources with no doubt.