There are two reasons why this feature was removed:
1. This feature was designed to let you change the table prefix to something other than jos_ which was used in Joomla! 1.5 and 1.6. Joomla! 1.7 and later use, by default, a random database table name prefix when installing. This made this feature obsolete. If you are using a third party installer which uses jos_ it is a major security bug in the third party installer and the proper long term course of action is getting in touch with its developers and ask them to fix it.
2. The proper way to change the database prefix without risking bricking your site is:
- Take a backup with Akeeba Backup
- Restore the backup. In the Database Restoration page choose a different prefix.
- Delete the tables with the jos_ prefix from your site using phpMyAdmin, Adminer or any similar tool provided by your host
If you are wondering why using Akeeba Backup is the recommended way, it has to do with PHP timeouts and foreign keys. Admin Tools is a Joomla! component. This meant that it has to change all database tables' names at once, otherwise it would result in an unusable site. However, if the PHP timeout limit is reached before that process is complete it does result in an unusable site. Furthermore, the process will fail if there are foreign key relations between tables. Akeeba Backup is designed to overcome this and many other issues MUCH BETTER than Admin Tools. In fact, the only reason it can work around these issues is that the restoration script (ANGIE) is a standalone PHP application which doesn't rely on the Joomla! site's tables. This means that we can perform a staggered database restoration (many page loads) without resulting in a bricked site.
TL;DR Use Akeeba Backup to backup and restore your site, changing the database prefix in the Database Restoration page. It is the most stable and recommended method.
Nicholas K. Dionysopoulos
Lead Developer and Director