Changing Hostname to different domain

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http://forums.cpanel.net – Hi, I currently have my server with a hostname vps.domain.com, how can i change the hostname to for example vps.newdomain.com? When i have tried to do this i am able to change the hostname and reach the login page for WHM/Cpanel at vps.newdomain.com but it says my credentials are invalid even though they are correct. Thanks Jamie (HowTos)