How do I determine what is different between two different xlsx versions of the same worksheet?

I apologize in advance if my question seems somewhat vague (if only I could attach a file that would be so much better!) I use TestComplete for regression testing, part of which is .rdl-type reports in our software being exported in the .xls format and compared to a previously saved copy. A newer version of the software will export the file in the .xlsx format, and pretesting this in TestComplete is causing the following issue: The .xlsx file opens just fine using Microsoft Excel, but the following in TestComplete fails to open it: DDT.ExcelDriver(strSourceSpreadsheet, strSheet, True) However, if I open the file with Microsoft Excel and re-save it (making no changes), then the above line of code works just fine. So the next thing I did was to change the .xlsx extension on the before Excel and after Excel versions to .zip, and then tried comparing the two collections of files. The re-saved version is almost twice as big as the original one, has additional files, and all the common files are full of differences. How to I even begin trying to get to the bottom of this? Has anyone experienced something similar? MyAccountAccess Online