Maintaining a consistent release version across all SEPMs and SEP 12.x clients in an organization is not a requirement for support. It is supported to have newer SEPMs managing older SEP 11 clients, and older releases of the SEPM should be able to successfully manage SEP 12.x clients that have a newer version of the software. These configurations are not recommended, but can arise as complex organizations perform testing and trials in accordance with their change management policies.
In the event that an older client is managed by a newer SEPM, the client only adhere to policy features that the client has. In the event that an older SEPM is managing a newer client, new client features will not be configurable from the SEPM. Client-Server communication will work to report client status, however version inconsistency between SEPM and Client may result in some features being partially functional or completely unavailable. Best practice is to upgrade the SEPM first, and then upgrade clients to the same version as soon as possible.
Please note that there are instances where the latest release is necessary on both SEPM and SEP 12.x client to completely correct a known issue. Symantec Technical Support can provide details and will recommend upgrading both SEPMs and SEP 12.x clients in these instances.
Reference: http://www.symantec.com/docs/TECH231546