I just went through along delay with DNS when updating our Company' DNS when changing Public IP addressing scheme. I am also in the process of migrating from SBS 2003 to windows 2008 and exchange server 2010. I am use the link below as a guide;
I am at the point ready to migrate mail boxes to the new server once I migrate the mail boxes I need to change my MX record and reverse look up in DNS again. Demazter say's to leave both exchange server in place for a couple of days before doing the uninstall of Exchange on the SBS Server. Should I change my MX record and reverse look up in DNS immediately after migrating the the mail boxes or should I wait until I am ready to uninstall exchange on the SBS Server. How every I do it the MX record and DNS record change over will be done Friday so as to minimize problems.