Change Domain for Control Panel

I am struggling with updating the domain to the control panel for an EE site. The old domain which the control panel references is no longer registered and therefore gets a redirect. I’ve poked and prodded through the forums a bit and it looks like I should be paying attention to the

$config['cp_url'] =

line. I’ve modified it accordingly but still no dice. Other things I’ve attempted are modifying the index.php file and the

$system_path =

line.

Any help would be much appreciated!