The aluminum may be tough, but I've broken a couple of them by trying to force them off when stuck on with years of ungodly crud. What happens is the outermost "lip" that curls around the locking pin can snap off.
Also just a word of caution in removing the bag -- again, if it's really stuck on there you can risk tearing the fabric by forcing it, especially with older Kirbys. What should help is to dump the Sani-Emptor in a bucket of lukewarm water with the bag attached, and let it soak a while. The water should loosen the gunked up gunk and make it easier to get the bag off, but again, go SLOWLY and CAREFULLY. There are fewer heart-sinking sounds than that "krrrrstch" of the bag ripping apart....... "Been dere, done dat."