I was having the same problem with my multi-city itinerary. At the time of booking I had upgraded the outbound leg to F and a couple of days later wanted to change the return. Since I couldn't get an agent through the US toll-free number I tried the UAE number. After about 10 minutes I got an agent who changed my return leg. When I asked why I couldn't make the change myself, she said it was because there was an upgrade on the first leg. Not an entirely intuitive nor necessarily satisfying answer, but there you go.