Changing main domain for multisite

Ok, this might seem like a noob question but I'm a bit confused here.
I've got a wordpress multisite installation and I've also got installed Multi-domains and wpmudev's domain mapping plugins.
In multi-domains I've got domain1, domain2, domain3 etc.
Now domain1 is the main domain. So if you try to go to the url of domain2 you're redirected back to domain1.

How can I change the MAIN domain so that instead of domain1 it's domain2?

And yes, I know this isn't exactly related to the multi-domains plugin, I was just wondering if there will be any implications I haven't thought about.