Ailrines that allow leg changes on awards without canceling
 

We almost never fly round-trip, preferring open-jaws instead.. but the actual itinerary sometimes needs to be worked out later, while availability is fleeting.
In this age of married segments and dynamic pricing, it's even tougher than usual, but which airlines allow changes in segments of the award without canceling the whole itinerary (which may or may not come back)?
In other words, if there is a booking for, let's say JFK-ADD-JNB, JNB-ADD-JFK, and I would like to change one of the legs to ADD-ZNZ or similar (with availability), which airlines allow that without canceling JFK-ADD segment?