[Hampshire] rackspace debain IP change

A client has askedif we can help migrate a rackspace machine IP address
handling thier email.

Evidenly it has two IP addresses available, They want to stop using the
existing IP address and mail domain and start using a new domain and
IP address.

As I have no experience of using rackspace systems does anyone have any
suggestions or advice? At the memnet even getting into the rackspace
user interface is proving problematic :-(

Jacqui