Make multisite sites unique hosts

Have your DNS people setup a wildcard pointing to the WordPress server. Make sure the server can recognize the sub-domains or configure it for those.

Leave the root ‘master’ or network account alone and go into network from there to sites and edit each site with the new URL…

So /Site30 becomes site30.mymultisite.com respect the present format of each entry.

Do a backup of everything just incase something breaks.

If you really need to change the network account then talk with your host or support people first. That can be easy and can be perilous territory.