New Post: Convert/Save HSSFWorkbook as XSSFWorkbook

I'd like to know, if there is a way to save a HSSFWorkbook as XSSFWorkbook (and vice versa)
I tried the following, but it's not working:

HSSFWorkbook lXlsWorkbook = new HSSFWorkbook("c:\file.xls");
MemoryStream lMs = new MemoryStream();
lXlsWorkbook.write(lMs);
lMs.Position = 0;
XSSFWorkbook lXlsxWorkbook = new XSSFWorkbook(lMs); // Here the error occurs "wrong header"
FileStream lFs = new FileStream("c:\file.xlsx", FileMode.OpenOrCreate, FileAccess.Write);
lXlsxWorkbook.write(lFs);
lFs.Close();
lMs.Close();

Has someone done this before? Kind regards, Thomas