Hi,
in my opinion this plan to migrate a single SEPM in a middle size environment is OK on the paper but not less risky than other more comfortable procedures. A lot of effort has been put to implement in the product a migration wizard from 11.0 to 12.1 as much simple, safe and automatic as possible while your manual activities to move settings and clients from a SEPM to another look to me like a source of headache, maybe more stress, hence higher risk of human mistakes.
A different approach could be:
1) take a proper backup of your SEPM 11.0:
http://www.symantec.com/docs/TECH102333
2) restore it in a separete test network, you need to keep the same IP and hostname:
http://www.symantec.com/docs/TECH102333
for a realistic simulation, install a test SEP client too but with the sylink.xml of the production installation, check if the connectivity is OK, you will then know if you are really able to recover your SEPM in case of issues
3) test the migration and the product in the test environment
4) once you are more comfortable with SEP 12.1, migrate only the SEPM in production (after fresh backup), no consequences are expected in the clients since product version and settings are kept but, in case of issue, you already know how to roll back to SEPM 11 (step 2)
5) once anything is OK with the SEPM, migrate only a limited number of clients from 11 to 12.1 for test, if anything is OK, move on with the rest
The advantages of the above procedures are:
- increase confidence in the disaster recovery procedure
- reduce the risk of human mistakes by using automatic procedures
- keep settings and logs with no effort
- put more focus on testing the product and the official procedures meant for it in regards of backup, recovery and migration rather than custom approaches.