You can access the original in Base.pi:
julia> pi
π = 3.1415926535897...
julia> pi = 3
Warning: imported binding for pi overwritten in module Main
3
julia> Base.pi
π = 3.1415926535897...
julia> pi = Base.pi;
julia> pi
π = 3.1415926535897...
julia> cos(pi)
-1.0
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