Windows - Upgrading to Server 2012

Hello we currently have 2 Windows Server 2008 R2 servers that both have the Active Directory Services installed on them. Server A is the server that is the primary domain controller. Server B is the other server.

From all the information that I have read online it seems that you have to do the following:

• 
  Run an adprep command line command that is on the Server 2012 installation disk on the domain controller (not sure if I am supposed to do this on Server A and B)
  Set up and join a NEW Server 2012 machine to the domain and give it the Active Directory Services and Roles
  Do some clicking and make the NEW server the primary domain controller

So basically my question is what happens to my 2 2008 R2 servers?
I would like to just pop the Server 2012 disk into them and upgrade the operating system but I am afraid of messing up the domain. If I have this 3rd machine as the primary controller temporarily and demote the 2 2008 R2 servers then upgrade them and then put them back on and then make Server A the primary domain controller again?
Do I need to remove the Active Directory Services and Roles from them and replace them with new machines?