If you want to run them at the same time as two different names, use the Mirror Domain feature.
If you want to drop (or redirect) the old one, start by setting up the new one as a Fully Hosted Domain under the same FTP user account. That user will have both domains in their directory. Then:
Rename (via FTP) the new empty one to example.sav and then rename the old one to example.com.
Once you're sure the new one is up and running, you can change the old one to a Redirect so it bounces the old visitors to the new site.