I dont know about migration from 2005 to 2k12 and running them parllely for a while then to 2k12 completly..
But i can give u a little knowledge about the assigning the IP addresses...
When any application or service or system holding an IP address in shuted down or terminated... The IP address will be remained available for any other device..
In Failover cluster manager we have an option called add another resource for particular service or group...
Once you shutdown the old device, IP address would be readily available.. as you can assign to any by selecting the option i mentioned above...
I am not sure wether i am 100% sure