Those kinds of arbitrary limitations to what changes you can do to your personal information on sites can be completely ridiculous, and they seem to be based on some theoretical static idea of what a user is (probably closer to a CS textbook example than a real human being).
Many sites that easily let you change your name doesn't let you change user names, if you happened to use your full name as your username then there's not much to do besides creating a new account. If you have a lot of data you want to keep and you can't migrate any of your data it's even worse.
Even if you're not changing your real-life name there are many valid reasons for why you might want to change your user name, I don't think I'm the only one who wouldn't want to use the user names I chose when I was 15.
There are many other examples of personal information you should be able to change, but so many sites restrict it. People change, people make mistakes. There are many times where I've realized I've made a typo when finally submitting some information regardless of how careful I was. There's no reason for this, other than laziness and lack of understanding of the user.
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We strive for transparency and don't collect excess data.