I have had to cancel two return flights recently. Neither can be cancelled online once you apply points, so you have to call.
One was out in paid Y with a points upgrade, and a paid J return, paid in part with a travel credit - I was passed around to 4 different people and in the end back to a senior person who sorted it in 5 minutes. About 30 mins on the phone.
Second was today. Out and back in paid Y, both with points upgrades. I was passed to 3 different people. In the end it had to go to a senior person again, who sorted it in 5 minutes. 20 minutes on the phone.
Its not a good experience. Its not the worse, but I really don't enjoy it.