Yes, but as has been mentioned, don't get caught with additional unregistered lower(s) that do not have an attached 16"> upper though. If you only have one registered lower then you are ok with several short uppers other than the length you listed on your form 1 or 4 & different calibers if you so chose. If it's gonna be a permanent length change other than what you've listed the ATF asks that you send them a letter advising such so they can annotate this in the registry.
"People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf." --George Orwell--