Hello,
In the Enterprise version, you can use Upgrade Clients with Package to upgrade existing clients:
However, the following cautions apply:
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If you upgrade from 11.x and use Application and Device Control, you must disable the Application Control rule "Protect client files and registry keys." After the clients receive the new policy, you may upgrade using AutoUpgrade.
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Due to possible bandwidth concerns, it is best to schedule AutoUpgrade for after hours. You can also stage and select a package on a web server when you run Upgrade Clients with Package, or you can use an alternate method to deploy the upgrade package.
AutoUpgrade is enabled by default for SEP SBE, but you can disable it. Go to the Computers page in the management console, right-click your Group, select Properties, and then click Disable Automatic Client Package Updates.
Reference:
Best practices for upgrading to Symantec Endpoint Protection 12.1.2
http://www.symantec.com/docs/TECH163700
Steps to prepare computers to install Symantec Endpoint Protection 12.1 client
http://www.symantec.com/docs/TECH163112
Hope that helps!!