Hi,
Thank you for posting in Symantec community.
I would be glad to answer your query.
I would suggest the following plan
1) Upgrade the SEPM only to the latest version 12.1 RU5.
2) Perform the Symantec Endpoint Protection Manager backup.
Follow this article to take necessary backups: http://www.symantec.com/docs/TECH160736
3) Uninstall SEPM
4) Uninstall SEP client
5) Upgrade Operating System
6) Install the same version of SEPM (12.1 RU5)
7) Follow disaster recovery by following same article: http://www.symantec.com/docs/TECH160736
8) Install the latest SEP client
9) Upgrade all the clients.