Upgrading servers to Server 2012

[alfatec]

We are currently running Ranger 8 on 2 Windows 2008 domain controllers. As part of our summer project we are installing virtualisation and will be

building 2 new server 2012 domain controllers, transferring all the roles from the 2008 DC's and then DCPromo'ing the 2008 DC's. What I am after

is what is the best way to transfer Ranger 8 to the new 2012 DC's. The 2008 and 2012 will be running alongside each other for a short while so

will I just be able to extend each one onto one of the new 2012 DC's or do you have to do a full transfer. Any advice or documentation would

be appreciated.