Hi,
First install SEP 12.1 on new machine. Create new package with the help of competitive uninstaller.
Competitive uninstaller utility could be found in the Symantec_Endpoint_Protection_12.1_Tools_and_Documents_EN.exe
After Unzip, it would be :
tools\Tools\NoSupport\CompetitiveUninstall
There is a PDF file & it will explain in details how to create competitive uninstaller package.
After installing competitive uninstaller package (it will remove SAV 10.1 and will install SEP 12.1) all the clients will point to the new server.
Monitor the situation for a week time.
If everything worked fine go ahead and decommission old SAV 10.1
I hope it will help you.