I am in the same position as Hitemstraight - wanting to change an existing Star Alliance reward booking to add on an extra segment that has just opened up (GRU-LPB). I am OK with cancelling, re-booking and paying the fees, but my big concern is losing the segments that I already have (YYZ-GRU and LIM-YYZ), since no extra reward seats are open for my YYZ-GRU flight.

I have heard various comments from Aeroplan agents on the cancel/rebook process - that it may take a few minutes to process, that the Aeroplan system may decide not to reopen/reoffer my cancelled segments (based on current load factors), and that even if the segments are offered someone else could snag them before I was able to rebook.

Can someone familiar with the booking systems clarify the steps and risks involved? (Empress?) If the chance of losing YYZ-GRU is high I may just buy a GRU-LPB ticket for $500. (The third alternative of spending 25K points on a one-way flight within South America seems unwise)