If you double click the file to where it allows you to edit the name and just add an "x" to the .xls, does it automatically convert?
No, changing the file extension will not convert an XLS file to an XLSX file.
If you were to try to open that file in a program that does not support XLS files, it would not open. Seeing as most programs that support XLSX files will also support XLS you likely would never encounter that situation. It is usually the opposite that is true...i.e. that not all programs that support XLS will also support XLSX (in particular much older versions of Excel).
It is the same basic concept of if I change the DOC or DOCX extension to say JPG and then try to open the file up in a pure image editor, it will not open.
The point is changing the extension of the file does not change the actual type of file...just want Windows THINKS the file type is. So, if the program does not actually support that file type, it will not open that file no matter what the file extension is.
Or do you have to open up the spreadsheet and SAVE AS and under the "save as type", select xlsx?