You will lose integration between Elements Organizer 11 and Elements Organizer 13/13.1.
Elements Organizer 13/13.1 is not just the latest version (new) but it is new in design. You can see the especially in its slideshow feature.
You have the opportunity to convert the 11 catalog in Elements Organizer 13/13.1, but you are forced to allow 13/13.1 to convert the old type slideshows to the new before it will allow them in the Elements Organizer 13.
You might want to get the perspective of the team at the Adobe Photoshop Elements Forum.