Multisite - Looking for ideas to best manage a main site change

I run about 5 sites on a multisite installation, the main one being under my main domain.

What I'm currently in the process of is creating a brand new site (with content changes etc) which will be on my main domain.

So I set about creating it as a new site with a subdomain. It's now finished and ready to go live. So in essence, I want to move newsite.domain.com to domain.com and move the existing site at domain.com to oldsite.domain.com.

I had no idea at the time that this would be such a pain in the ass to do.

Changing the sites shouldn't be a stretch for my skills but it's got me thinking about the best way to manage the sites. I have a few domains mapped for other sites and a couple of subdomains.

Whilst I'm going through this change I thought I'd ask others how they manage theirs and get some input. I've read every post and article on the net about how to change the domain of a multisite etc but nothing has quite answered my management questions.

To save the same thing happening in the future if I want to change the site, is it best to have my main domain for the Wordpress installation as a 'ghost' domain or one I don't use and have my main domain mapped using the plugin? This seems like the best way to be able to easily switch the site in the future (by changing the URL in the settings).

My main domain is also the main domain my hosting is running on, will this cause any issues or is there anything I need to look out for?

If I change the main domain of multisite, will it cause issues with my other sites running off subdomains of the current main domain?

Thanks for the time, any input/insight is valued. I've no idea the best way to do this right.