Going to be migrating two separate Windows 2003 AD Forests into a new 2008 AD Forest. One Forest is on Exchange 2003 and one is on Exchange 2007. Would like to move he new domain with Exchange 2010 - but one major application we run will not be supported by Exchange 2010 out of the gate (after discussing with the Vendor). It may be many months after the release of Exchange 2010 before it is offically supported. Which means it could be many months into next year before we could move to Exchange 2010.

Meanwhile the bulk of our users are on Exchange 2003 and we could benefit immediately with an upgrade 2007. One thought is to move forward and migrate to the new domain - but leave the Exchange infrastructures as they are - until we can move to Exchange 2010. The other school of thought is to move forward with the new domain migration and migrate everyone to Exchange 2007. What would be the major caveats of migrating to a new domain but leaving   This is the method I'm proposing.  To me the major caveat of leaving the Exchange environments as is is it greatly complicates the infrastructure - the old domains would need to be intact as well as the maintennance of the new domain. Every migration to a new AD domain I've heard of involves Exchange migrations at the same time. Trying to best justify the move to Exchange 2007 now (and upgrade to 2010 later) and showing the reasons why leaving old Exchange infrastructure intact is not the best idea.

Any thoughts, recommendations, links to similar case studies, etc. that may assist in bolstering our proposal?