As you have an external DB already, none of the long-winded migration articles are really needed here. Assuming you're happy for the new SEPM to have a different name and IP address you can just do the below:
- Install new SEPM as "Additional Server to Existing Site" (essentially creating a multi-SEPM site)
- Create a new Management Server List that tells your clients to talk to the new SEPM only and assign everywhere (allow time for clients to apply)
- Shutdown old SEPM
- Verify continued management and communciations with clients
- Log into new SEPM and delete old SEPM from the Site's server list
- Done
No messing around with backups or DR steps.
#EDIT#
Oh yeah, the bleow articles should aid in the above steps:
http://www.symantec.com/docs/HOWTO11097
http://www.symantec.com/docs/HOWTO81154
http://www.symantec.com/docs/HOWTO80735
(New MSL should list only the new SEPM)