We are in the process of switching ISPs to improve our bandwidth - service will overlap - we have not cancelled the first one but the new one will be providing service next week - they have provided me with my new block of static ip addresses.
Now internally I've got a Fortigate 80C firewall (V4MR2 - Patch2 ) so theoritically I could be supporting two wan interfaces (I would rather not have to figure out how to configure that and then put it back after we get rid of ISP 1 unless you give me a compelling reason to).
Behind the firewall are three servers - all running Server 2003; one is our primary domain controller and handles AV installations (attached to logon scripts) as well as hosting a web based paging service for our staff. Second server is Exchange 2003 (we are currently hosting our own mail although incoming is screened by third party SPAM Filter before delivery to us).
Now I know I need to reconfigure firewall with new addresses and I need to get the company who hosts our domain name to alter DNS records for us - which dns records do I have to request changes to and given the potential delay in propagation of dns changes how to I best time this to minimize disruption of service to all of our users (I can't afford to have 100+ users upset about missing e-mail!)