There appears to be a bug in changegroup.php if the username being modified has any quotes in the name, see the error below.
Code:
Database error in vBulletin 3.0.1:
Invalid SQL:
UPDATE user SET
usergroupid = 8
WHERE username = 'big_kev's_ss'
mysql error: You have an error in your SQL syntax near 's_ss'
' at line 5
mysql error number: 1064
Date: Monday 03rd of May 2004 08:59:16 PM
Script: changegroup.php