This is not something that's cleanly supported: We have to fully remove the addon domain, change the main domain, then add the other domain back as an addon. This will cause your document roots to change so your files will need to be moved, and anything else that may have been done to either domain (like setting up mailboxes, or changing PHP versions, etc.) will be undone.
My advice would be to leave it as it is.
Are you sure you want to do this?