At this time both the subdomain and the domain will have duplicate content and both will work. Once you have everything setup and running on the domain (ie you have all emails going to your new email accounts and the domain website is only used), you can then remove the subdomain from your server.
Thank you for your question NJMJdesign!
You can follow the steps below to minimize any downtime that you may have.
1. Create the Addon domain
2. Copy the files for the subdomain to the addon domain.
3. Create the email accounts for the domain.
4. Create forwarders from the subdomain email account to the domain email accounts.
At this time both the subdomain and the domain will have duplicate content and both will work. Once you have everything setup and running on the domain (ie you have all emails going to your new email accounts and the domain website is only used), you can then remove the subdomain from your server.
Regards,
James R