Logins and related things really do change, and for good reasons
Every so often it's popular to say that you will never, ever change a (Unix) login, an assigned email address, or whatever. No direct renamings, no new account to replace the old account, no nothing. Generally this attitude comes with a certain mixture of 'you should have got it right the first time' and 'if your login is less than ideal it doesn't really matter'.
This is wrong (and arrogantly blind). People periodically have excellent, compelling reasons to change their login et al and you are eventually going to have to change them one way or another. If you aggressively stick to your 'no changes' view, it's quite possible that very bad things will happen; one of the least bad ones is that important people will quietly leave your organization.
Let us take the most straightforward and obvious example. Suppose a married woman has a login, and of course when it was created it followed your common pattern of having her last name in it. Oh, and when she married she took on her husband's last name, because this is still common. Then one day she gets divorced and of course changes her last name back to her own. This is an excellent, compelling reason to rename her account, or rather two reasons at once. First, this woman is going to be want to be called (in logins, email addresses, etc) by what is now her actual name. Second, she is quite possibly not going to want to be reminded of her ex-marriage and ex-husband every time she logs in, gets email, has to send email, and so on.
If you tell this woman 'sorry, we're still not renaming your login, that's our policy', what you are doing is giving her a great big middle finger. If she has actual power in your organization, your policy is probably not going to last long and you will have created a bunch of bad blood. If she does not, any number of things may happen, such as her quietly resigning so she can go somewhere where she is not frequently reminded of her ex-marriage and how insensitive your organization is.
This is far from the only case where there are excellent reasons to change a login. It's simply an obvious one with a not uncommon situation where hopefully everyone can see the real injury done by not changing the login. People really do have really good reasons to change their login, reasons that they could not possibly have predicted in advance and so avoided. They are not being irrational or picky or any number of other things. And sooner or later you will wind up changing someone's login.
(In a relatively small environment it's possible for this to only happen very infrequently and you might actually never have it happen while you work for a particular place. In a large environment it probably happens relatively frequently.)
The corollary of this is that as much as possible you should design your systems so that they at least accommodate login changes from the start. Don't assume that logins, email addresses, names, and so on are unchanging. If you need an unchanging primary identifier for people, make it a meaningless one (a GUID or a random number is good).