I'm going to walk through the upgrade process in some more detail next, and also provide some general guidance afterwards about the Service Pack 3 installation steps as well as what to expect in terms of timing and service interruptions.
If you have an AD forest topology with multiple domains, or process restrictions that require schema updates to be managed a certain way, you can apply the Exchange 2010 SP3 schema update on a 64-bit domain controller that is in the same AD site as the Schema Master, using an account with Schema Admins and Enterprise Admins rights.
…During the upgrade, the database is dismounted, and all mailboxes in that database are taken offline.
This seemed to be a major issue to me until I performed the upgrade in my test lab.
This is a significant release that delivers some key functionality to customers such as support for Windows Server 2012, support for co-existence with Exchange Server 2013 CU1, and general bug fixes and security updates.
If you are planning to upgrade your Exchange 2010 servers to SP3 you should be aware that there is an Active Directory schema update involved.For Hub Transport, Edge Transport, and Unified Messaging servers there are no special steps required other than to manage your upgrades in a way that aligns with whatever high availability you have in place or those server roles.For example if you have two Hub Transport servers in a site, upgrade them one at a time.There is also be a “health checking” component added that’s already used by Windows Server 2008 which can check to make sure that certain upgrades and security features are in place for XP-based clients before they are allowed to log onto a network.Download it now to ensure that your Windows XP is fully updated.If that is a concern for your environment, but you still want the bug fixes and security updates, you might consider sticking with Service Pack 2 and applying Update Rollup 6 instead.At the time of this writing there are some points in the various release notes that aren't correct or fully updated yet that Microsoft are still working on or that are worth some clarification: You should also plan to update any management tools installations you have on admin workstations or servers, and also check your third party applications that integrate with Exchange in case they also need updated management tools.Otherwise the schema update will be applied when you upgrade the first Exchange server.Client Access servers are the first server role to update, and you should begin with the internet-facing site if you have multiple sites in your organization.To continue, verify that the file exists, and either grant administrator permissions to the account currently running Setup or log in with an administrator account. Hate to answer my own question, but I had to manually un-install Client Tools, re-install them using SQL Server 2005 setup, and THEN apply the updates.It wasn't working by just trying to apply updates over the existing original installation.