I am retiring my old Windows Server 2003 Standard 32bit machine and migrating to a faster and more powerful new machine with Windows Server 2008 R2 Standard.
The two sticking points are migrating user accounts (and there are a lot of them) and IIS settings/websites (again, there are a lot).
The new machine has not been provisioned yet. I'm at that point where I'm about install the OS on it. The old machine is configured with a mirrored set for its OS and data partitions. I have broken the mirror set, replicated all of the data to an external drive, and then rebuilt the mirror set. In short, I have an image of the old machine to play with while safely leaving it up and running.