How do I change the hosting type of a domain from being forwarded to being hosted locally on the server without affecting things like the domains email accounts?
If you change the hosting type to "forward" all files in the document root will be deleted. It's not a suited methode if you wanter to perform maintenance on a website. Instead configure forwarding with .htaccess (or nginx directives).
@jnasser to add: You cannot change the hosting type of your main domain if add-on domains exist in the subscription. As Kaspar suggests, add a line to .htaccess like