Hello,
In your case - there are 2 scenarios -
1. Upgrade the SEPM 12.1.2 on the existing 2k3 server to SEP 12.1.4, then move the installation to the new 2k8 server by performing a disaster recovery.
How to move Symantec Endpoint Protection Manager 12.1 from one machine to another
http://www.symantec.com/docs/TECH171767
Symantec Endpoint Protection 12.1: Best Practices for Disaster Recovery with the Symantec Endpoint Protection Manager
http://www.symantec.com/business/support/index?page=content&id=TECH160736
OR
2. Move the installation of SEPM 12.1.2 from the existing 2k3 server to the new 2k8 server, then upgrade it to SEP 12.1 after it's moved.
How to move Symantec Endpoint Protection Manager from one machine to another
http://www.symantec.com/docs/TECH104389
How to point clients to a new SEPM after decommissioning or replacing the primary SEPM.
http://www.symantec.com/docs/TECH92556
Migrating from Symantec Endpoint Protection v.12.1.2 clients to Install Symantec Endpoint Protection v.12.1.4 could migrate flawlessly without any issues.
It is always recommended to perform a Disaster Recovery for SEPM 12.1.2 before Migrating to SEP 12.1 as a Backup step.
Here are few steps to look at for Migration:
1) How to upgrade the Symantec Endpoint Protection Manager (SEPM) to Version 12.1 RU1
http://www.symantec.com/docs/TECH176260
2) Upgrading and migrating to Symantec Endpoint Protection 12.1
http://www.symantec.com/docs/TECH163602
3) Steps to prepare computers to install Symantec Endpoint Protection 12.1 client
http://www.symantec.com/docs/TECH163112
4) Activating your Symantec Endpoint Protection 12.1 product license
http://www.symantec.com/business/support/index?page=content&id=TECH162623
Also, check the Articles for the Migration on :
Quick Access to Symantec Knowledgebase Articles of Symantec Endpoint Protection 12.1
https://www-secure.symantec.com/connect/articles/quick-access-symantec-knowledgebase-articles-symantec-endpoint-protection-121
WhitePaper for Migration: https://www-secure.symantec.com/connect/downloads/migration-whitepaper-symantec-endpoint-protection-version-121
Hope that helps!!