It is unlikely that there will ever be an upgrade path with wizards, etc. to perserve settings and migrate things cleanly. Retaining your data isn't likely to be an issue; you should be able to wipe your current system partition, install Windows Server
2008 on that partition, and not affect any data. You will be walking away from all the wizards in Windows Home Server, though, so no more client backup, no easy media sharing, remote access, etc.
I'm not on the WHS team, I just post a lot. :)