Hi,
Prior to an upgrade you should have clear picture about ports which SEP 11 uses.
Which Communication Ports does Symantec Endpoint Protection 11.0 use?
http://www.symantec.com/business/support/index?page=content&id=TECH102416
Ports and Protocols to be allowed when using a proxy in a SEP environment.
http://www.symantec.com/docs/TECH131843
Second thing, you should have done an upgrade to SEP 12.1 instead of SEP 11.
Check this article: http://bit.ly/JpDR3U
In current situation could you try to replace sylink.xml on new clients & check whether it works or not.
Restoring communication to clients with a new Sylink.xml file
http://www.symantec.com/business/support/index?page=content&id=TECH106288