there are systems with case-sensitive usernames and short ago there was even a strange guy explaining that e-mail addresses are case-sensitive for him and me@mydomain.tld is another person than ME@myadomain.tld
Fascinating! Thanks for the explanation of the login_lc option too. The update SQL "UPDATE users SET username = LOWER(username);" is ok if there aren't duplicates yet, but I suspect it will produce problems if there are duplicates (i.e., it will select the first of these).
Mirko