Change domain for primary site and a few sub-sites only

Hi,

I have a multisite install with about 25 sites set up as mydomain.com/site1, mydomain.com/site2 and so on. I now want to change the domain for my primary site to mynewdomain.com and I want 3 sub-sites to be mynewdomain.com/site1, mynewdomain.com/site2 (and so on) but the rest to remain under mydomain.com/... as they are today.

What is the best practice to achieve this?
I have the plugins "Domain Mapping" and "Multi-Domains for Multisite" installed.

Thank you in advance!