Update windows 2003 domain controller to Windows 2012

Hi all,

I have two windows 2003 domain controllers, I am planning to update one of windows 2003 domain controller to windows 2012 domain controller first. 

I am concern is that how windows 2012 domain controller work with old windows 2003 domain controller before I update the second old windows 2003 domain cotroller.

Becasue then, I should waiting some time after update first old windows 2003 domain controller. Finally, I will update second one.

Can any one has a good plan to update two windows 2003 domain controller to windows 2012 domain controller? thanks a lot.