You can do this with a redirect. You will need to go to the old site’s cPanel interface and then go to ‘Redirects’. Set the old site’s index page to redirect to the new site’s index page.
Alternatively, if you wish for the old site and the new site to go to the same place but have not set up the new site yet, you can use a Parked Domain. A Parked Domain allows you to add a second domain name to your account that goes to the same website. For example, if you currently have johnsforums.com and you wish to add the domain cityeyez.com which also goes to the new cityeyez.com site, simply add johnsreallycoolforums.com as a parked domain and both domains will go to the same place.