I have done what you are planning to do. And, I will be doing more of what you are planning to do. I have 80+ BE servers across the WAN. Don't be surprised if you get errors if you don't update the agents. I have been working with Symantec support to fine tune our best practice for migrating from 11d 6235 on Windows Server 2003 to 11d rev 7170. The best practice includes pushing the agents, rebooting the agents after the push, and creating all new selection lists, policies, or jobs. When this is followed, it takes a long time, but the sailing is smooth in the long run.