Agree with above instructions, and here's why.
Even if you uninstall the software - either via the Partner Management Console / Hosted Endpoint site - you are going to have to run the Symantec clean-up tools to ensure you have a clean environment for the installation of SEP 12.1.6.
But I don't depend on the Symantec clean-up tools. After they run I will manually delete the following folders if they still exist:
C:\Program Files\Norton* and/or Symantec*
or
C:\Program Files (x86)\Norton* and/or Symantec*
C:\ProgramData\Norton* and/or Symantec*
and then run CCleaner to ensure that all Norton and Symantec Registry entries are eliminated.
After all of that I will reboot one last time before attempting the installation.
And no, Symantec, doesn't make this process easy. Although I wish, for at least one second, they would.