Migrating Backup PDC and UCS server type

Hello, Im just planning to migrate a Windows server 2012 environment to UCS with windows clients, and linux NAS

I have a PDC and a backup PDC, the backup is used when maintenance happens on the PDC the backup is turned on to provide services to keep the network alive, AD is replicated between the PDC and the backupPDC

So for the migration the PDC needs to be a Primary Directory Node
But the backup PDC Im confused, should that be a Replica Directory Node or a Backup Directory Node, the latter seems to have to require a complicated and irreversable set up to transfer role to become the Primary Directory Node.

Does UCS offer a hot standby PDC?

Many thanks