This document details the supported upgrade paths to Windows 7. Best advice: don’t do in-place upgrades. Migrate user data (if it’s stored on the PC – should be on a server), rebuild using WDS, MDT or ConfigMgr and then restore user data.
EDIT #1:
And just as I publish this post they also release a document on Windows Server 2008 R2 upgrade paths. OK, remember this is only an x64 OS. No in-place upgrades from x86. No in-place upgrades from Full to Core installations or vice versa. You also can’t change languages in an upgrade.
MS strongly recommends against in-place server upgrades. The only acceptable scenario is when a server is stable and has 100% MS components. But even then, all the advice is migrate, not upgrade. I’ve got a chapter on this subject in one of the Mastering Windows Server 2008 R2 books. It was 65 pages when I submitted it to the editors/reviewers last week …. before screenshots.