Mac MVP James Gordon has posted directions on how to convert old Excel (.xls) files to standard (.xlsx) files using Automator.
What does this converter do?
You choose one (.xls) file, several (.xls) files, or an entire folder filled with (.xls) files and the converter will save a copy of each Excel file in standard open XML file format, better known as the “new” (actually more than 5 years old now) Excel (.xlsx) format. This converter works with old Excel workbooks that do not contain macros.
Requirements: This workflow requires Microsoft Office 2011 Home & Business. The Office 2011 Home & Student version does not contain Automator workflows.