On the "manage domains" page in the panel, you should still see the expired domain. The piece of information you need from that is the USER name the the old domain name was attached to.
Now, on the same page "manage domains" add the new domain name as "fully hosted" and use the SAME user the old domain used.
Wait a few minutes, until the directory for the new domain is automatically added by the panel robot. You can tell when this step has been completed by FTP, SFTP, or shell access. You're looking for the new almost empty directory for newdomain.com that will be automatically added for you.
Now depending on your skill level, you could 'rm -rf' that new directory and rename the old to the new. Or if you are just using ftp it might be easier for you to just use rename to swap the two names.... i.e. rename newdomain.com to TEMP then rename olddomain.com to newdomain.com and then finally rename TEMP to olddomain.com
That will get FILES associated with the old domain name served from the new name. If you are working with a web app such as wordpress, you'll likely also have some database changes to make to get everything working correctly again.