MemoryStream

var
  US1, US2: UTF8String;
  MStream: TMemoryStream;
begin
  US1 := AnsiToUtf8(Memo1.Text);
  Memo1.Clear;
  MStream := TMemoryStream.Create;
  MStream.Write(PUTF8String(US1)^, Length(US1));
  SetLength(US2, MStream.Size);
  MStream.Position := 0;
  MStream.Read(PUTF8String(US2)^, MStream.Size);
  Memo1.Text := Utf8ToAnsi(US2);
  MStream.Free;