DouKaku?

using System;
using System.IO;
using System.Text;

class Program {
    static byte[] SjisToEuc(byte[] bytes) {
        return Encoding.Convert(Encoding.GetEncoding("shift-jis"), Encoding.GetEncoding("euc-jp"), bytes);
    }
    static void ConvertFile(string path_in, string path_out) {
        using (StreamReader sr = new StreamReader(path_in, Encoding.GetEncoding("shift-jis")))
        using (StreamWriter sw = new StreamWriter(path_out, false, Encoding.GetEncoding("euc-jp"))) {
            while (!sr.EndOfStream) sw.Write((char)sr.Read());            
        }
    }
    static void Main(string[] args) {
        // buffは、Shift-JISで "日本語文字列"
        byte[] buff = new byte[] { 0x93, 0xFA, 0x96, 0x7B, 0x8C, 0xEA, 0x95, 0xB6, 0x8E, 0x9A, 0x97, 0xF1 };
        byte[] result = SjisToEuc(buff);

        Console.WriteLine(BitConverter.ToString(result));
        // output : C6-FC-CB-DC-B8-EC-CA-B8-BB-FA-CE-F3
        
        ConvertFile(@"D:\sjis.txt", @"D:\euc.txt");
    }
}