Have a read of this:

Migrating to Symantec Endpoint Protection 11.0 MR3  Article URL http://www.symantec.com/docs/TECH105919

The migration plans says to upgrade the Manager first, then the clients can be upgraded.

Overview of the migration process
Migration to the current version of Symantec Endpoint Protection includes the following steps in order:
 

• Create a migration plan
  Before you begin to install the Symantec Endpoint client, manager, and any administration upgrades, you should have a solid understanding of your network topology and a streamlined plan to maximize the protection of the resources on your network during the upgrade. Symantec strongly recommends that you migrate the entire network to the current version rather than managing multiple versions of Symantec Endpoint Protection.
• Backup up the database
  Before you upgrade, you should back up the database.
• Disable replication
  If your site uses replication, you must disable replication before upgrading Symantec Endpoint Protection Manager. You must disable replication at each site that replicates.
• Stop the Symantec Endpoint Protection Manager service
  Before you upgrade, you must manually stop the Symantec Endpoint Protection Manager service on every management server in your site. After you upgrade, the service is started automatically.
  WARNING: You must stop the Symantec Endpoint Protection Manager service before you perform this procedure or you will corrupt your existing installation of Symantec Endpoint Protection Manager.
• Upgrade the Symantec Endpoint Protection Manager
  You do not need to uninstall management servers before you install the new version. The overinstall process saves legacy settings, and then upgrades to the latest version.
• Enabling replication after migration
  After you migrate all servers that used replication including the servers that were configured for failover and load balancing, you need to re-enable replication. After migration, you add a replication partner to enable replication. You only need to add replication partners on the computer on which you first installed the management server. Replication partners automatically appear on the other management servers.
• Upgrade the Symantec Endpoint Protection Clients
  You do not need to uninstall previous clients before you install the new version. The overinstall process saves legacy settings, and then upgrades to the latest version.