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I'm planning on migrating from Exchange 2010 to Exchange 2013 and understand that I need Exchange 2010 Service Pack 3 in order to coexist with Exchange 2013. That said, I'm curious as to exactly why we need coexistence periods and what they generally consist of.
First off, while Exchange 2010 Service Pack 3 (SP3) does allow for coexistence with Exchange 2013, you should understand you also need Cumulative Update 1 for Exchange 2013.
It's best to refrain from maintaining multiple versions of Exchange in the same organization for too long.
Note: Exchange 2007 SP3 rollup 10 also allows for coexistence with Exchange 2013; Exchange 2013 CU1 is necessary here as well.
Coexistence periods are necessary during Exchange Server migrations (typically in larger organizations) so that the migrations are executed as smoothly as possible. For example, if you are migrating from Exchange 2007 to Exchange 2013 and have a large number of users (say, over 10,000), it's highly unlikely that you would migrate all of them over the course of a single weekend.
Additionally, as Microsoft continues to develop Exchange Server, and more features are introduced, changed or deprecated, it can take time to make the required modifications to your environment. In order to do so, trickle-in servers running the version of Exchange you're upgrading to are often needed to take on specific responsibilities. For example, you may need to introduce the Exchange 2013 client access server role to take over OWA and Outlook client requests for your entire environment.
While there's no general rule as to how long that your coexistence scenario should last, it's best to refrain from maintaining multiple versions of Exchange in the same organization for too long. Doing so introduces unnecessary administrative complexities, and can further complicate things. Coexistence should only last as long as it takes to get mailboxes, associated services and resources over to the version you're upgrading to.
About the author
Andy Grogan is a multiple recipient of the Microsoft Exchange MVP award (2009-2013). He is based in the U.K. and has worked in the IT industry for the last 16 years, primarily with Microsoft, HP and IBM technologies. Grogan's main passion is Exchange Server, but he also specializes in Active Directory, SQL Server, storage solutions, technology strategy and technical leadership in large-scale enterprises. Grogan currently works for a large county council in Surrey as its technical delivery manager and supports 15,000 customers on more than 240 sites. Visit Andy's website at www.telnetport25.com/.