Even when you change the hostname in cPanel and Plesk, it doesn't necessarily change inside of Linux. This can cause all sorts of issues and headaches when you're trying to tighten authentication (email, etc) and various rules and tasks.
Edit
/etc/sysconfig/network in your favorite editor. (I use nano.)
Code:
nano /etc/sysconfig/network
Comment out whatever is there currently, and add a new entry:
Code:
HOSTNAME="server.new.com"
#HOSTNAME="server.old.com"
NETWORKING="yes"
And that should do it.
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