If I am already registered or a principal, do I ever have to file a complete application and verification in ORS instead of a "short" form?

Yes. The only time that this will happen is the first application you file in ORS. For example, if you have been registered continuously as a AP with your current firm since before May 2002 (when NFA adopted the electronic requiring requirements) and are now adding  a principal status with that firm, you will need to file a complete application.