Best Approch to Upgrade 2008R2/2012 cluster in windows 2012 R2 Cluster

Hi Team,

I want to consolidate my Windows Hyper-v server/cluster into one single Windows 2012R2 cluster .

i have below environment.

1. Three  Windows 2012 R1, Clustered Hyper V.
2. Three Windows 2008 R2, individual Hyper V host. Most of the Virtual Machine are running with RDM disk.

can any one suggest me best approach to upgrade all three windows 2008R2 standalone host into 2012R2 and than consolidate into single cluster along with windows 2012R1.

can i convert RDM into iscsi Volume.

Thanks

Ravindra

Ravi