using System.IO;
using System.Text;

namespace Convert1251Utf8
{
    class Program
    {
        static void Main(string[] args)
        {
            string text = File.ReadAllText(args[0], Encoding.GetEncoding(1251));
            File.WriteAllText(args[1], text, Encoding.UTF8);
        }
    }
}

            using (var srcStream = File.OpenRead(args[0]))
            using (var srcReader = new BinaryReader(srcStream, Encoding.GetEncoding("windows-1251")))
            using (var dstStream = File.OpenWrite(args[1]))
            using (var dstWriter = new BinaryWriter(dstStream, Encoding.UTF8))
            {
                const int BUFFER_SIZE = 50000000;
                var chars = new byte[BUFFER_SIZE];
                int charsRead;
                while ((charsRead = srcReader.Read(chars, 0, BUFFER_SIZE)) > 0)
                    dstWriter.Write(chars, 0, charsRead);
            }

            var bufferSize = 5120000;
            using (var srcReader = new StreamReader(args[0], Encoding.GetEncoding("windows-1251"), false, bufferSize))
            using (var dstWriter = new StreamWriter(args[1], false, Encoding.UTF8, bufferSize))
            {
                string line;
                while ((line = srcReader.ReadLine()) != null)
                {
                    dstWriter.WriteLine(line);
                }
            }