Any specific reason as to why the MMS is running BE 2012 when the CASO is BE 15 ?
Would recommend the following steps -
1) Upgrade the MMS to the same version as that of CASO
2) Make the MMS server a standalone server by removing it from the CASO server.
3) Stop all services on the MMS server and copy the Data & Catalog folders.
4) Install BE (same version) on the new server.
5) Stop all services and replace the Data & Catalog folders. If the server name is different, a SQL cmd is to be run to update the server name.
6) Restart all services.
7) Join this new server to the CASO.
Alternatively, if the other server already has BE installed, then it becomes more simplified.
Run Backup Exec Utility and choose "Move a managed Backup Exec server." option.