Yes, you can, but you can not take any advantage SEP 12.1 features, why bother to upgrade the clients, please upgrade SEPM and then client.

Thanks. Can I install SEPM12 over existing SEPM11 installation? Will SEPM12 be able to manage SEP clients version 11 after the upgrade?

Hello,

Yes,You can directly upgrade from 11.x to 12.1 RU1 and SEPM 12.1 can managed 11.x clients .

 Obtaining the latest version of Endpoint Protection or Network Access Control

http://www.symantec.com/business/support/index?page=content&id=TECH103088&actp=search&viewlocale=en_US&searchid=1316154271551

Upgrading or migrating to Symantec Endpoint Protection 12.1.1000 (RU1)

http://www.symantec.com/business/support/index?page=content&id=TECH174545

Activating your Symantec Endpoint Protection 12.1 product license

http://www.symantec.com/business/support/index?page=content&id=TECH162623

Your satisfaction is very important to us.If you find above information helpful or it has resolved your issue...please mark it accordingly :)

If the SEPM is 11 & clients 12. You will see the clients in SEPM, but the definitions will not be updated.

If the SEPM is 12 & clients are 11. no worries. It will be good.

Agree with NRaj.

Upper SEPM version to lower client version will manage sucess but lower SEPM version to Higher client version will not work

Definitely upgrade, but do so with thought and planning.  Here are a couple of good articles:

Planning for Migration to Symantec Endpoint Protection Manager 12.1
Article: TECH162730   |  Created: 2011-06-20   |  Updated: 2011-08-01   | 
Article URL http://www.symantec.com/docs/TECH162730 
 

How to Install Symantec Endpoint Protection v. 11.x from the Symantec Endpoint Protection Manager v.12.1?
Article: TECH174099   |  Created: 2011-11-10   |  Updated: 2011-11-14   | 
Article URL http://www.symantec.com/docs/TECH174099