Hello,
Are you planning to upgrade you SEPM and all client from 11.x to 12.1 or just one win 7 client to 12.1?
AS SEPM 11.x will not be able to manage 12.1 client.
You can install SEP 12.1 on win7 machin as unmanaged client as well.
If you are palnning to migrate,your migration path should be as below
11.x ---> 12.1RU1--> 12.1 RU1 MP1
Upgrading or migrating to Symantec Endpoint Protection 12.1(RU1 )
http://www.symantec.com/docs/TECH174545
Upgrading or migrating to Symantec Endpoint Protection 12.1.1101 (RU1 MP1)
http://www.symantec.com/docs/TECH187753