Great post from Rhoderick Milne explaining the design options when moving from a legacy Exchange Server to Exchange Server 2013.
When designing an upgrade strategy from an older version of Exchange to a newer one, a question that needs to be addressed is do we need to introduce a version of Exchange that may not currently be present? This may be when upgrading from Exchange 2003 to Exchange 2010. If that organisation does not have any Exchange 2007 servers, you need to evaluate if there may be a future requirement for one in the future. Examples include:
Application specific requirements
Client versions in use
Backup/Restore software requirements (though can be met with a separate recovery forest)
Once that first Exchange 2010 server is installed it is way to late to go back and introduce Exchange 2007. Actually its before the installation, but hold that thought for now. The same is also true when upgrading from Exchange 2007 to Exchange 2013, if there are no Exchange 2010 servers in the organisation.
Let’s look at an example where we are upgrading from Exchange 2007 to Exchange 2013.
Read more at source: http://blogs.technet.com/b/rmilne/archive/2014/02/06/point-of-no-return.aspx