Multisite override current blog on load of Wordpress (URL)

I am not sure if I should post this here but I advent had much response from Wordpress.

I have been attempting to use sunrise.php to override the current multisite blog. I have tried : wp_cache_switch_to_blog( $blog_id ) though this has no effect.

I have also tried everything in this tutorial, but I think this is out dated as it using site instead of the blog.

What is the correct way to override the current multisite blog?

Or could some point me in the direction of which core file handles setting it?