function ConvertCodePage(const S: RawByteString; CodePageIn, CodePageOut: Integer): RawByteString;
begin
  Result := S;
  SetCodePage(Result, CodePageIn, False);
  SetCodePage(Result, CodePageOut, True);
end;

var
  s_in, s_out: RawByteString;
  f_in, f_out: TMemoryStream;
begin
  f_in := TMemoryStream.Create;
  try
    f_in.LoadFromFile('c:\Source\Delphi\ConvertCP\Win32\Debug\ansi.txt');
    f_in.Position := 0;
    SetLength(s_in, f_in.Size);
    f_in.Read(s_in[1], f_in.Size);
  finally
    f_in.Free;
  end;
  s_out := ConvertCodePage(s_in, 1251, 65001);
  //s_out := UTF8Encode(s_in);
  f_out := TMemoryStream.Create;
  try
    f_out.SetSize(Length(s_out));
    f_out.Write(s_out[1], Length(s_out));
    f_out.Position := 0;
    f_out.SaveToFile('c:\Source\Delphi\ConvertCP\Win32\Debug\utf8.txt');
  finally
    f_out.Free;
  end;
end.

function ConvertCP(vCodePageIn, vCodePageOut: Integer; const pXML_DATA: WideString): UTF8String;
begin
  if (vCodePageIn=1251) and (vCodePageOut=65001) then
    Result := UTF8Encode(pXML_DATA)
  else
    raise Exception.Create('Convert from '+IntToStr(vCodePageIn)+' to '+IntToStr(vCodePageOut)+' not implementing yet');
end;