HELP!

I'm a comparative newbie... I rent a dedicated server off fasthosts.co.uk running a standard LAMP system, but when I set it up, I set it up with the wrong domain name as the first domain... I then changed the dns settings so that domains on the server have the dns ns1.newdomain.com and ns2.newdomain.com... however, localhost is still olddomain.com, so when postfix issues a HELO it does so with olddomain.com

This hasn't caused a problem, but I'm now trying to set SPF records for the mail servers, and I'm pretty sure the problem I'm having stems from that issue (the reason being that I have another server with same set up, but the dns and local host are the same and I was able to make the changes to the BIND DNS files absolutely fine and have the SPF verification checkout fine)

HOW can I reset the localhost? to match the primary domain? ... I've attempted several changes so far to no avail (though I haven't killed the server :-)
1. changed etc/hosts
2. placed newdomain.com before olddomain.com in the named.conf file
3. edited the etc/postfix/main.cf

Any ideas, short of full backup and restarting? Unfortunately, the server is live with stuff that is important to me

Adam (with fingers crossed)