Change main site / network admin site?

I have a multisite setup using domains.

• Main/First Site = mydomain.com
• Second Site = www.mydomain.com
• Third Site = blog.mydomain.com

All my main site network admin is on mydomain.com I would like to:

1. move the site network admin access URL to say wpadmin.mydomain.com
2. retire all content on mydomain.com

this is because inside my network mydomain.com resolves to my domain controllers... i am using hosts file to override that for now, buts that not sustainable.

Is this possible without doing this Switching primary site in WordPress Multisite (for example i note in WP there is now a dropdown to change primary site, i note this didn't change main site - so i am little confused)