Why should usernames be unchangable?

Question

I read everywhere (and see in practice) that usernames should not be changeable. When I ask why, \'security\' is given as a reason.

I\'ve been searching for a definitive

Answer 1

I disagree with the premise. There are plenty of big name sites that let you change your username; eBay is one of the first ones that comes to mind.

Edit:

Now that I think about this, and read Rook's answer, I remembered about a place I worked at where you could not create an account with a username that already existed, but you could always change it (later, after registration) to a username that already existed. Unfortunately, at said place, permissions were based on the username... you can see where the security problem lies.