Hi,
As pete_4u2002 said Sylink replacer will be one of the easiest option to point clients to new server.
Only one point to add here, SEPM 12.1 can manage both SEP 11.x and SEP 12.1 clients but SEPM 11.x can manage only SEP 11.x clients.
To replace sylink.xml file you would required Sylink replacer tool, there are two different Sylink replacer tools one for SEP 11.x & other for SEP 12.1
To get sylink replacer tool you will have to call support or log a web case.
please contact Symantec Technical Support via the support phone numbers listed below
Regional Support Telephone Numbers:
United States: https://support.broadcom.com (407-357-7600 from outside the United States)
Australia: 1300 365510 (+61 2 8220 7111 from outside Australia)
United Kingdom: +44 (0) 870 606 6000
Additional contact numbers: http://www.symantec.com/business/support/contact_techsupp_static.jsp
To create a web case :
QuickStart Guide - Create and Manage Support Cases in SymWISE
http://www.symantec.com/docs/HOWTO31132
How to update a support case and upload diagnostic files with MySupport
http://www.symantec.com/docs/TECH71023
I hope it will help you !!!