Ok.
(1A) By changing CName, my users will be able to put the new files or edit current files from the new 2012 server right? Even if the old server is still in the network (but demoted already).
(1B) I did some googling for the CName, and went into this guide,
http://technet.microsoft.com/en-us/library/ff625726(v=ws.10).aspx
Is this referring to what you recommended me to do?
(2) I did some reading up on Step-by-Step Guide for Setting Up Windows Server 2012 Domain Controller - TechNet Articles - United States (English) - TechNet Wiki, why is this guide showing FFL, DFL as Windows 2008 R2 and not 2012 (even though the guide is for 2012 server)