Hi ductTape!
We changed a our IP/ISP a while back. My suggestions are more towards the ISP.
- Let your users know there is a chance email received during this transition may be lost. Though most systems will resend to your addresses, it is possible.
- Speak with your new ISP about the day and/or time of change, how long it will take them to update/change your MX records. You don't want them to give you the, ". . . we can make our changes anytime between 10am and 5pm" answer. Ideally, you want to make your changes on the server, while on the other line you tell your ISP, "OK. Make the changes now." Some MX records can take longer to propagate throughout certain routers (EIGRP/OSPF v2) and could actually take up to 24-48 hours for a complete update.
-- meaning someone in Utah may have to wait a day or 2 before their emails get through to your people.
- Find out what your new IP ranges are (if any). Even though you are more than likely only going to use one, usually they will assign 5 IPs for you. This is good to know for the future, and later down the road, if you look to activate/use those addresses.
Good Luck with your switch.