We are aware that there's an issue when/if you at the moment change your Twitch username.
I'm here to say that @Crosseye_Jack is working on a fix for it.
You should still be able to read this even if you're not logged in. So if this has happened to you do let us know on Twitter. You can either get at us @visiblespeech or @CrosseyeJack. While we can't do individual cases (due to security risk, since we can't confirm you're the owner of the old/new account) we would still like to know how's effected so we reach out and check when we have a fix.
I will remove this topic once it's resolved.