I had this exact same problem with 3000+ users all in the moderators group.
I did the following MySQL line and moved them all to the users group, took 5 mins, saved me 5 months of hand editing!
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UPDATE `database_name`.`ubbt_USER_GROUPS` SET `GROUP_ID` = '4' WHERE `ubbt_USER_GROUPS`.`GROUP_ID` =3
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The only thing you need to edit in the code above is to put your database name where it says "database_name" ... the rest of the code should be standard on normal installations. This will move all your users from the Moderator Group (Group 3) to the Users group (group 4).
