$update['errors'] .= '<p>Please make the names unique and inform the users in question about the changes.</p>';
Hmm, I don't like this proposal. If once the user-name was valid, it should be valid all the time (like the Password: md5 --> sha1).
Please note that we are talking about a bug here. If the forum has two users Daniel and Däniel, only one of them will be able to log in to the forum. This is how I discovered the problem.
I think that the forum in the past had Latin1 encoding but this was changed to UTF-8. (I might be wrong, since I'm new to MyLittleForum). And with Latin1, Daniel and Däniel is two different user names.
With utf-8 encoding and the use of the collation utf8_general*, the MySQL database treats similar characters as the same character, for example A and Ä. And therefore, Daniel and Däniel is now the same user name.
If two users has the same user name, one of them will not be able to log in, so this issue must be resolved.