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Earlier last year, we announced that Exchange 2010 Service Pack 3 would be coming in the first half of 2013. Later, we updated the timeframe to Q1 20...
Updated Jul 01, 2019
Version 2.0
Blog Post
According to the documentation, you don't need 2013 CU1 for Coexistence:
Exchange 2013 Coexistence support
You must install Exchange 2010 SP3 if you want to run Microsoft Exchange Server 2013 in a coexistence mode. You can't perform an in-place upgrade from Exchange 2010 to Exchange 2013. However, you can install an Exchange 2013 server into the existing Exchange 2010 organization after you install Exchange 2010 SP 3. For more information about how to install an Exchange 2013 server into the existing organization, see Install Exchange 2013 in an Existing Exchange 2010 Organization.
After you perform this procedure, your organization will be running in a coexistence mode. You can maintain this mode indefinitely, or you can immediately complete the upgrade to Exchange 2013 by moving all resources from Exchange 2010 to Exchange 2013, and then decommissioning the Exchange 2010 servers. For more information about how to upgrade to Exchange 2013, see Upgrade from Exchange 2010 to Exchange 2013.
It's also confirmed by a co-worker who performed a migration from 201 to 2013 in the same AD.