I like to share that in Aspose.Slides there are two separate API’s dealing with PowerPoint 97-2003 format and PPTX format. The namespace Aspose.Slides is responsible for managing all presentation formats supported in PowerPoint 97-2003, that includes PPT, POT & PPS. While, namespace Aspose.Slides.Pptx is responsible for managing PowerPoint 2007-2010 supported formats like, PPTX, POTX & PPXS. At the moment the inter-conversion of these two formats is not supported in Aspose.Slides and we are working in this direction. Hope it answers the query for you. Please share, if I may help you further in this regard.