I just did this and it is great that they allow it. In my situation, I wanted to change my return, but it was outside the advance purchase window, so I would have lost my internet one-way if I cancelled my reservation on the outbound (you can't partially cancel an itinerary until at least one leg is flown). The one-way on the return would have been full fare. Instead of being able to change me return to a fare restricted to RT, I had to book an entireley seperate RT. I only flew one leg on each itinerary. It would be great if you could change one leg without losing your fare.

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Randy