When using this tool to change the table prefix from wp_ to sometingelse_ then the usermeta keys wp_capabilities are not changed to reflect this. The result is that all users are locked out. This can be resolved by using phpMyAdmin or some other database tool.
But the plugin authors should reaaly fix this.
This plugin cannot be recommended until this is fixed. The option ot change the table prefix should be removed until it does the job properly.