Given that the login name seems to be a “primary key” throughout the website, I guess it would be impossible to change them while the site is up. But maybe it could be done during site downtime, although then of course, there are usually other more serious things to worry about…

And then of course there are all the inbound links that could get broken.

Is this a can of worms that has been looked into?

I haven’t tried it in a while, but the last time I did, the account in question “broke” and traumatized me