One of the big questions, when considering migrating from Server 2003 to Server 2012 R2 is whether your should upgrade your existing Active Directory environment, or whether you should migrate to a new Active Directory environment.
As with many other big questions in IT, the answer to this comes do to “it depends”
The benefit of migrating to a new forest is that it gives you a chance to go back and fix things or change decisions that were made when your organization first deployed Active Directory. If you’re still on Server 2003, it’s fair to say your organization probably isn’t a fast mover. A fair number of organizations have the forest structure that they put in place when they first adopted Active Directory, either when they deployed Windows 2000 for the first time or when they deployed Windows Server 2003.
The decisions that were made about that deployment were likely to have made sense when that deployment occurred. However whether or not they still make sense in 2015 is quite another matter.
Just as we still use the QWERTY keyboard, which was designed to fix a problem with speed typing on old manual typewriters, it’s possible that you’re stuck with a forest structure that made sense to someone more than a decade ago, but doesn’t reflect the realities on your organization today.
So migrating to a new forest allows you to create a forest that suits your organization is it is now. It also gives you a chance to clean out some of the cruft, such as derelict accounts and objects, that may have built up since Active Directory was originally deployed.
The drawback to performing a migration, rather than an upgrade, is that it requires a substantial amount of planning. This is a drawback because we’re only about 4 months away from Windows Server 2003 end of extended support. If you haven’t done the planning now. Well. It’s sorta like starting reading “A Newbie’s Guide To Using A Parachute” after the airplane’s engines have stopped running and everything is about to go pear shaped.