New to Java

Hello All,
I was using POI 2.5.1 to generate an XLS report. All was fine until the problem cropped up when the Report generation in Production crossed the 65k+ limit set by Excel 2003. Since anyway we open the reports in Excel 2007, my immediate instinct was to change the filename of my report from .xls to .xlsx. Obviously this didn't work since I was using POI 2.5.1.

Then I downloaded the latest POI 3.6 and tried to open the generated .xlsx file. Even now I get the same error as I used to get with POI 2.5.1 as follows:
"Excel cannot open the file '<filename.xlsx>' because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file."

Moment I revert the file extension to .xls, it allows me to open the file. Please let me know your thoughts.

I just checked the Apache documetation and found that 3.6 is compatible with JDK 1.5 or higher. I generally compile with 1.4. In the process of altering this. Does this really make a huge difference?

Regards,
Raghu