Use phpmyAdmin to backup the following tables:
 jos_core_acl_groups_aro_map 
 jos_core_acl_aro
 jos_users

*Note "jos_" will equal "mos_" if you are using MAMBO
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Relationships
JOS_users (KEY FIELD =ID) important field ('usertype')
usertype will be one of..

Registered
Author
Editor
Publisher
Public Backend
Manager
Administrator
Super Administrator

Each one of those possible MEMBER GROUPS is represented in the following file as follows
jos_core_acl_aro_groups


Key field here is 'group_id'

MAKE A NOTE OF THE GROUP_ID for the 'Author' and the 'Registered' member types (!IMPORTANT)
We want to change occurences of Author to Registered

Take note of  jos_core_acl_aro
Each member has an entry in that file and each user has an aro_id that is the field that ultimates determines what type of user it is, as it links into the following file jos_core_acl_groups_aro_map

finally,
jos_core_acl_groups_aro_map

Key fields: the aro_id which points back eventually to the actuall USER
Group_id: which points back to the actual usertype in jos_core_acl_aro_groups


So your UPDATE statement (if you are daring enough to have proceeded this far) will change all the group_id in jos_core_acl_groups_aro_map from AUTHOR_Code to REGISTERED_code

 

TAKE YOUR JOOMLA INSTALLATION OFFLINE before you apply the changes.