In my case I am migrating two 2003 DHCP Servers (that are also DCs) with split scopes over to two 2012 R2 DHCP Servers that are load balanced
Before you Migrate
Windows Server 2012 R2 DHCP Expired Leases Not Getting Scavenged worth following suggestions to potentially avoid the issue
Migration
Steps to follow for migrating DHCP Server to Windows Server 2012 R2
After Migration
Managing a DHCP failover cluster static reservations , relevant for Windows Server 2012 and 2012 R2
Windows Server 2012 R2 DHCP Expired Leases Not Getting Scavenged keep an eye on this