Mac MVP James Gordon has posted directions on how to convert old (.ppt) files to standard (.pptx) files using an Automator workflow.
What does this converter do?
You choose one (.ppt) file, several (.ppt) files, or an entire folder filled with (.ppt) files and the converter will save a copy of each PowerPoint file in standard open XML file format, better known as the “new” (actually more than 5 years old now) PowerPoint (.pptx) format. This converter works with old PowerPoint presentations 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.