Changing subdomain and ip

[ask2me007]

Dear all
Iam using zimbra for the last 2 years. It goes smootlhy because of the tremendous help getting from the members of this form.

My domain is forwarding all mails to our mail server using a subdomain . As my ISP is unable to increase the bandwidth of my ip, I have to opt for another ip having high bandwidth. During this change, I also want to change subdomain from pop.xxx.com to mail.xxx.com.
What care should I ave to take before doing this??
What are the changes to be made in my mailserver for that??

[ask2me007]

Any body pls help me??