Thanks for your interest in Aspose.Slides.
I regret to share with you that Aspose.Slides
for Java does not currently support inter-conversion between PPT and
PPTX or vice versa. An issue with ID SLIDESJAVA-17175 already exists in our issue tracking system as a new feature request. You will be updated via this forum thread once this feature is available.
We are sorry for your inconvenience.