I just had to make a change for another outbound flight (in addition to the one mentioned above) without changing the return flight (and my upgrade).

I tried to do this on-line and it looked good but the web site wouldn't take my (tiny) payment successfully. The original itinerary was untouched after checking again.

I called in and the first agent told me that it was not possible to do this without "exchanging" the entire ticket (and losing my upgrade on the return)

I called back and the next agent made it happen just fine, and my return flight and upgrade were not touched.

Still some SHARES education needed, I'd say!