“Can I upgrade to Windows Server 2012?”
Yes. But it depends.
On what you’re upgrading from, and what your hardware supports. But even if you don’t do an upgrade in place, a migration is completely do-able with the right tools.
FREE tools. (http://technet.microsoft.com/en-us/library/jj134039.aspx)
“And what if I’m upgrading an Active Directory domain controller? What happens then?”
What have you done to move to Windows Server? Do you have any methods, tips, or tricks that have worked well for you? Any “gotchas” that you’d like to share? Please do!.. in the comments.