Is it possible to migrate a Windows Standard 2003 installation to a Windows Standard 2003 R2 installation.
We have a Domain controller that is currently on 2003 but wish to take advantage of some functionality that is only available in the R2 edition. We have the necessary licenses and media but not sure if it is possible or indeed how to go about doing it.
It's definitely possible. Windows Server 2003 R2 was a new release of the Windows Server 2003 OS, but, so long as you're licensed to use it, the actual installation is nothing more than an add-on to the base Windows Server 2003 OS (unlike W2K8 R2, which is a new release of the OS). Microsoft has documentation here and here that ought to be of help to you (the second link speaks to the specific details of upgrade from one type of product key to another and the restrictions therewith).
Basically, you're going to verify you're at the proper service pack level for whichever install procedure apples, then perform the "disc 2" setup (installing the R2 components). You're not going to have to reinstall the OS or rebuild the server. It's really fairly painless.
I just did this last night actually. Here are the steps I took.
Obtain Server 2003 R2 disc number 2 with product key for R2, of the same flavor as your current install. (Standard, Enterprise, etc. This is important)
Run adprep /forestprep from the disc \CMPNENTS\R2\ADPREP to make the forest R2 ready
Install the R2 update
Re-Activate windows with new key.