Hi @SterlingF,
I marked @Jeff_F's reply as the solution here because this really is how Canvas is designed. When a course is concluded/ended, the course is basically in an archived read-only type state. I know the nickname is a slightly more personal thing, but as as admin, I can certainly see a slippery slope happening if nickname changed were allowed for concluded courses. I think Jeff's suggestion of setting a reminder for yourself to change the nickname before the end of the course is good. You could also ask your local admin if they would be willing to change your old nicknames for you (but it would involve temporarily changing dates on all of your old courses at least temporarily, so I wouldn't be surprised if they say no).
Sorry for any inconvenience this is causing you.
-Chris