Symantec Endpoint Protection 12.1.5xxx (RU5) can upgrade seamlessly over the following:

    Symantec Endpoint Protection 12.1.4100.4126 (RU4 MP1), 12.1.4104.4130 (RU4 MP1a), and 12.1.4112.4156 (RU4 MP1b)
    Symantec Endpoint Protection 12.1.4013.4013 (RU4) and 12.1.4023.4080 (RU4a)
    Symantec Endpoint Protection 12.1.3001.165 (RU3)
    Symantec Endpoint Protection 12.1.2100.2093 (RU2 MP1)
    Symantec Endpoint Protection 12.1.2015.2015 (RU2)
    Symantec Endpoint Protection 12.1.1101.401 (RU1 MP1)
    Symantec Endpoint Protection 12.1.1000.157 (RU1)
    Symantec Endpoint Protection 12.1.671.4971 (RTM)
    Symantec Endpoint Protection 11.0 (can be upgraded to enterprise version only)
    Symantec Endpoint Protection Small Business Edition 12.0

Note: A Small Business Edition product can be upgraded to the enterprise version, but the reverse is considered a downgrade and is not supported.

Migration from Symantec AntiVirus 10.x or Symantec Client Security 3.x is not supported. You must uninstall these legacy products before you install Symantec Endpoint Protection 12.1.5


Install Notes

1. Unpack & Install as unmanaged client.

New fixes and features in Symantec Endpoint Protection 
http://www.symantec.com/business/support/index?page=content&id=TECH224706