StreamReader read = new StreamReader(@"D:(path)");

StreamWriter write = new StreamWriter(@"D:(path)");

Encoding ANSI = Encoding.GetEncoding(1252);
Encoding UTF8 = Encoding.UTF8;
byte[] utf8_bytes,ansi_bytes;

utf8_bytes = UTF8.GetBytes(read.ReadLine());
ansi_bytes = Encoding.Convert(UTF8, ANSI, utf8_bytes);

string ansi_str = ANSI.GetString(ansi_bytes);

write.WriteLine(ansi_str);

read.Close();
write.Close();

StreamReader read = new StreamReader(@"D:(path)");

StreamWriter write = new StreamWriter(@"D:(path)");

Encoding ANSI = Encoding.GetEncoding(1251);
Encoding UTF8 = Encoding.UTF8;
byte[] utf8_bytes,ansi_bytes;

utf8_bytes = UTF8.GetBytes(read.ReadLine());
ansi_bytes = Encoding.Convert(UTF8, ANSI, utf8_bytes);

string ansi_str = ANSI.GetString(ansi_bytes);

write.WriteLine(ansi_str);

read.Close();
write.Close();