SEPM 11.x to 12.x upgrade - Older version cannot be removed

Trying to upgrade SEPM 11.0.4202_MR4_MP2 to latest 12.1.5.  Towards the end of the install process, it shows an error:

"The feature you are trying to use is on a network resource that is unavailable:  \\server\Software\Symantec EndPoint Protection\Symantec_Endpoint_Protection_11.0.4202_MR4_MP2_AllWin_EN_CD1\SEPM\"

It is looking for the original 11.x setup files, which were long ago removed.  After clicking cancel, the upgrade fails with the message:

"Error 1714. The older version of the SEPM cannot be removed.  Contact your tech support group."

How can we resolve this?  I've looked on FileConnect for the 11.0.4202_MR4_MP2 original download and cannot find it.