So we are moving servers and DB backend as well...
Take your backups of course :)
I would think the easiest (especially for large number of clients) way would be to upgrade the current SEPM to 12.1.4 (and DB schema will be upgraded as well)
Then install the second/new SEPM as a replication partner to the 2012 Server, and the replication will go right to a SQL server - once the replication is complete - let a couple days pass to make sure that all the clients receive the newer communications file (that will list the old server AND the new server)
Once a few days pass (if you wait longer [say a week or even two]), you can change the management server lists to point only to the new server, remove the old server from the replication/partnerships then offline the old server...
This is the least messy, and least likely to orphan clients...sure restoring their communications is easy, the only problem is figuring out which machines are missing (especally if you are dealing with hundreds and moreso if thousands)
However, since you are migrating/upgrading a database from 11.x to 12.1 - PLEASE review ALL your policies, quite frankly, I would suggest recreating new policies for AV, Firewall, Application and Device Control, and LiveUpdate - the default settings that come with version 11, are not the best for 12.1 or modern networks/machines/etc - you can use your old policies as a reference but I wouldn't use them for long term...