Changing primary domain without touching e-mails

So my deal is: I have a client with domain.com as the primary domain. He also has a few subdomains and e-mail accounts in this domain.com. Now he wants to change his primary domain to OLD.domain.com, and add domain.com as subdomain(?), so he could have control (via cPanel) over the destination which is domain.com pointing to on the server (/public_html/whatever_he_likes/). So my question is: how to change primary domain on the account from domain.com to old.domain.com (yes, the subdomain of domain.com), and not to change all other subdomains he already has (example.domain.com will become example.old.domain.com, which I want to avoid), and not to change domains of e-mails he already got (mail@domain.com will become mail@old.domain.com - also want to avoid). OR maybe there is another solution for Client to have full control over the root directory of primary domain? (I've changed it manually for now via editing /var/cpanel/userdata//).