To migrate to a different mail server or IP address, you do not need to (and should not) change your MX entries. All you need to do is change the delivery location used by the filtering service. Within the Management > Inbound filtering > Mail Delivery page, simply specify the new mail server name or new IP address. If in doubt, we recommend using the IP address rather than the name of the server.
This change typically takes less than fifteen minutes to propagate throughout our servers.