I don't think, you could do this using a GPO. Version 11 has lot of differences, and you would have to manually perform the upgrade.
The upgrade process is not very difficult though:
1. First install you Ver 11 Server, which is called the Symantec Endpoint Protection Manager.
2. Once it is installed, create client install packages, and deploy to you Ver 10 clients, which will automatically upgrade.
3. Also deploy a SEP client on your Symantec Endpoint Protection Manager server computer.
Kindly refer to the following :
http://www.symantec.com/business/support/index?page=content&id=TECH102397&actp=search&viewlocale=en_US&searchid=1296040690250