Because there are features in the PPT API (Presentation, Slide, etc) that are not in the PPTX API (PresentationEx, SlideEx etc) I decided to see if I could open a PPTX file and convert it to PPT then carry on using the PPT API. It seemed like I could, because there is an enum value to export to PPT, but it gives a runtime error that it's not implemented - see attached screen shot which also shows the code I was using.
Is there another way to accomplish this?