Hi John,
I would really not recommend it. There is some procedure to upgrade from CentOS 6 to CentOS 7, but there is too much chances to break something. You're better to hire a new server with cPanel and CentOS 7, then do a migration to the new server and discontinue the old server once everything is done.
WHM Transfer tool is now very well done. You can transfer all the account and also the server settings (EasyApache, Exim, for instance).