Hello,
A Similar issue has been rectified in the Latest SEP 12.1 RU1, check the Release Notes Article:
http://www.symantec.com/docs/TECH174565
"Error 1920. Service Symantec Migration Service (SEPMasterServiceMig) failed to start" when upgrading from SSEP 5.1 client to SEP 12.1 client
Fix ID: 2439998
Symptom: When upgrading clients from SSEP 5.1 to SEP 12.1, an intermittent error occurs. This error occurs when an older version of the dll remains on the system and is attempted to be loaded by a newer version of the driver.
Solution: Changed the upgrade logic to rename sysfer.dll on upgrade, preventing this error.
I would recommend you to Migrate to the Latest Version of SEP 12.1 RU1.
Hope that helps!!