Please Help: Error when table prefix changed.

hi. i hope you can help. i am trying to change the table prefix in my wordpress installation since i read online that this is necessary to prevent hijacking. my wordpress version is 2.6.5.

i set my permissions for wp-admin.php to 775 and i followed the ff. steps diligently:

1. backup your wordpress database to a sql file (you can use phpmyadmin)
2. open that *.sql file (make another copy first) using text editor, then find and replace all “wp_” prefix to “something_”.
3. now, drop all tables of your wordpress databases (don’t drop the database)
4. import the *.sql file which has been edited before into your wordpress databases.
5. and lastly, edit your wp-config.php file and change the $table_prefix = ‘wp_’; to $table_prefix = ’something_’;
6. you may find that your plugins are deactivated automatically when this happens, so you’ll want to activate them again if that’s the case… I’d recommend deactivating them prior to doing this anyway as a precaution.

so i thought everything should be fine…but then when i accessed my admin page, it returned the ff. error:

Warning: Cannot modify header information – headers already sent by (output started at/home/myusername/public_html/mydomainname.com/blog/wp-config.php:63) in /home/myusername/public_html/mydomainname.com/blog/wp-includes/pluggable.php on line 770

i also tried installing wp security scan to do the renaming automatically but it does not work. it tellsme the username i am using does not have adequate permissions although i have given my username all the rights available.

i can no longer access my admin page.

what should i do next? please help. it is greatly appreciated.