// fix the usermodel container
_slides.add(newIndex, _slides.remove(oldIndex));
// fix ordering in the low-level xml
List<CTSlideIdListEntry> slideIds = _presentation.getSldIdLst().getSldIdList();
CTSlideIdListEntry oldEntry = slideIds.get(oldIndex);
slideIds.add(newIndex, oldEntry);
slideIds.remove(oldEntry);
}