it's simple,
option 1)
Back up of existing SEPM DB and certificates ( check these links).
upgrade the OS to Windows 2008.
Restore the SEPM DB after builind SEPM ( DR steps).
Then upgrade the SEPM's to 12.1 RU 1.
option 2)
Back up of existing SEPM DB and certificates ( check these links).
Then upgrade the SEPM's to 12.1 RU 1.
Back up of upgraded SEPM DB and certificates
upgrade the OS to Windows 2008.
NOte: have the same SEPM hostname and IP address
DIsaster Recovery Links
SEP 11 – Disaster recovery
http://www.symantec.com/business/support/index?page=content&id=TECH102333
SEP 12- Disaster recovery
http://www.symantec.com/docs/TECH160736
Upgrade links
Upgrading or migrating to Symantec Endpoint Protection 12.1.1000 (RU1)
http://www.symantec.com/business/support/index?page=content&id=TECH174545
Installation and Migration Documents for Symantec Endpoint Protection 12.1
http://www.symantec.com/business/support/index?page=content&id=TECH163707
Planning for Migration to Symantec Endpoint Protection Manager 12.1
http://www.symantec.com/business/support/index?page=content&id=TECH162730