Hi everyone,
I thought I would just share my limited experiences on this subject. When SARS broke out I was able to make quite a few changes to a few Aeroplan Star tickets with any problem. However, those were done under quite extraordinary circumstances. Had the itineraries not been changed, my whole family would have been quaranteened for days. The concierges at YVR were particularly helpful. They even allowed date changes in addition to routing changes. ^ All in all, I agree with previous posters that unless it's something exceptional it's essentially not possible to make any changes. Redepositing the miles seems to be the only way.