In general, I am not expecting username changes to be as frequent as the general worry would suggest. Though it's an apples to oranges comparison because changes aren't freely available, the number of username change requests and re-registrations (to get a different identity) combined is extremely small, and they're quite infrequent.
We keep, and will continue to keep, a full log of all such changes, and will be keeping an eye on it as we review the change in policy.
There is, additionally, a username reuse delay, which prevents new users from registering with or changing to a previously used username for 1 year.
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Sylas There is already a limitation in place that all usernames must be unique (the comparison is case-insensitive). There's no plan on username change rate limiting at the moment.
An update for private tags will be a separate release - I'm trying to keep this one as self-contained as possible in the event of an issue. Changing private tags would mean reparsing the entire database of posts to retroactively adjust them, and it's not simple to just roll back from that.
Your emails will remain private as they currently do. They're just used for logging in, and as a point of contact for any thread subscriptions, notifications, password resets, etc.
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AsenRG Anybody on your ignore list remains on your ignore list (and the same with the friends list). That, like the vast majority of things, is based on userid.
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Cleric8092 Thanks for bringing that up - the new registration system is not bringing the restriction on yahoo emails with it, pending review, of course.