You should do a forum search for "civil aggression", lots of good stuff lurking here.
German Shepherd Dog Forums - Search Results
Some people do mix the two. Depends on the person, depends on the dog. One is a structured sport, the other is a very different form of protection work (may or may not include muzzle work, bite suits, hidden sleeves, etc).
I think how hard it is to switch depends on the innate drives, temperament and traits of the dog itself.