Hello,
Correct, It would migrate over the SAV 10.x, you may not require uninstallation of SAV 10.x unmanaged client.
Migration detects and migrates installations of the following Symantec legacy virus protection software:
• Symantec AntiVirus Corporate Edition 9.x and 10.x.
• Symantec Client Security 2.x and 3.x
• Symantec Endpoint Protection Small Business Edition 12.0
• Symantec Endpoint Protection 11.x *
• Symantec Sygate Enterprise Protection *
• Symantec AntiVirus for Mac *
See also Planning for Migration to Symantec Endpoint Protection Manager 12.1
Reference:
Upgrading or migrating to Symantec Endpoint Protection 12.1
http://www.symantec.com/docs/TECH163602
Hope that helps!