I ran across this article today while I was looking for something else :) and thought I would share it with you. 

Windows Server 2008 R2 Upgrade Paths

A couple things to note.  You cannot upgrade from a Server 2008 Core installation to a Server 2008 R2 full install.  Core to Core upgrades only.

Moving from Server 2003 requires SP2 for 2003.

Upgrading from x86 to x64 is not supported.  No surprise here, but just a reminder.

I’ve upgraded some of my Server 2008 servers to Server 2008 R2 without issue, it took considerably longer than a fresh installation, but I did get to keep my configuration settings by doing the upgrade.

One last thing to keep in mind: Server 2008 R2 is only 64 bit, so none of your 32 bit installations have an upgrade path to R2.  If you have a 32 bit installation on 64 bit hardware, you’ll still need to do a fresh install.

Until next time!

Rob