With our upcoming IP address changes, we are having a number of customers asking about how to keep their site functional when the old IPs drop off.

The simple answer: DNS TTL.

Before the upgrade, simply lower the TTL on your DNS records to something low (say 15 minutes, or 900 seconds).  After the upgrade, restore the TTL to its previous setting (say, 28800 or 8 hours).  Doing this, users should be directed to the wrong IP for no more than 15 minutes.

We will attempt to implement this for records on our nameservers on the behalf of customers.