Replacing one value in a large csv of floats with a string

General Usage
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2

You could use a Union type for the column to make assignment of a string a valid operation:

julia> df = DataFrame(a = 1:3, b = rand(3))
3×2 DataFrame
 Row │ a      b
     │ Int64  Float64
─────┼─────────────────
   1 │     1  0.644675
   2 │     2  0.754814
   3 │     3  0.458067

julia> df.b = convert(Vector{Union{Float64,String}}, df.b);

julia> df.b[2] = "zeroish"
"zeroish"

julia> CSV.write("test.csv", df)
"test.csv"

julia> readlines("test.csv")
4-element Vector{String}:
 "a,b"
 "1,0.6446749255742438"
 "2,zeroish"
 "3,0.4580669558867503"
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