Considerations for changing Leased Line supplier

We will shortly be changing our leased line provider & I want to be sure that everything is in place to ensure a smooth transition. Could someone please point out what areas I need to consider. It will be a managed router with a new range of external IP Addresses for my external facing servers etc, though these have not yet been assigned.

These are the things that I am aware of so far:
1) NAT in my firewall - add in the new external IPs when known
2) DNS in my AD (2003 Server) - change to the name servers for my new ISP?
3) Domains (e.g. mycompany.co.uk) - change the IP addresses in the A records for my mailserver etc…

I’m not quite sure what I need to change in my DNS, so some help there would be appreciated.

For my domains, can I setup secondary records so that my new IP addresses & my existing IPs are valid?

Is there anything else I need to consider?

Many thanks in advance.

For your DNS-Servers in your AD you have to change the IPs of the external DNS servers.
You find those IP adresses under the FORWARDING tab in the properties of your DNS server in the DNSMGMT Snap-In of you MMC.
As you can still reach the dns servers of your old ISP this change is not that critical, only performance may be degraded a little bit.