The domains' registration at the registrar has nothing to do with their status in cPanel. What nameservers are you using for the addon domain? If you are using the current main domain's nameservers, you will need to change the nameservers that are used to host the domain. Otherwise, nothing in cPanel will be affected by the main domain expiring.
If you want to use the current addon domain as the account's main domain, and completely remove the current main domain, you can do the following:
- Back up the account in question before making any changes.
- In the account's cPanel, remove the addon domain. This will not remove the subdirectory that contains the addon domain's site content.
- In the WebHost Manager, use Main >> Account Functions >> Modify an Account to change the domain name associated with the account in question.
- Move your site content from the addon domain's subdirectory to the public_html directory.
- Recreate any e-mail addresses and FTP users that belong to the domain that was the addon domain.
Again, I strongly advise backing up the account before proceeding. You can do this from the shell as root with the following command:
where "cpanelaccount" is replaced with the actual cPanel account username.
# /scripts/pkgacct cpanelaccount