I would think that you have to save the .xls in a system that will accept is
as .xlxs
or, if it contains macros, .xlxm and then do the import.

"Nate" wrote:

I'm importing a large number of Excel 2003 files into a SharePoint document
library. I'v written a utility to do this. If possible I'd also like to
upgrade them from .XLS to .XLSX.

Does anyone know how to use the Excel API to do this? Thank you very much