Quickly replace first line of large file

I have many large csv files (1-10 gb each) which I'm importing into databases. For each file, I need to replace the 1st line so I can format the headers to be the column names. My current solution is:

using (var reader = new StreamReader(file))
{
    using (var writer = new StreamWriter(fixed))
    {
        var line = reader.ReadLine();
        var fixedLine = parseHeaders(line);
        writer.WriteLine(fixedLine);

        while ((line = reader.ReadLine()) != null)
            writer.WriteLine(line);
    }
}

What is a quicker way to only replace line 1 without iterating through every other line of these huge files?