Manual:MigrateUserGroup.php

Details
migrateUserGroup.php file is a maint-scripts>Special:MyLanguage/Manual:Maintenance scripts|maintenance script to re-assign all users from an old group to a new one. This script is e.g. helpful after you renamed a user group in LocalSettings.php and you want the users, who were member of that group before, to stay in it also after the renaming. Another usecase is when you want to remove a group from all members.

Should a member of the old group also already be a member of the new group, the script takes care to remove the old group from his user account.

Only works on explicit user groups, meaning on those groups, which are actually stored in the  table inside the database. Does not work with implicit groups like "*" or "user", which are not stored in the table. This means the script currently cannot be used to e.g. add all users from the "user" group to another group.