Microsoft has published an excellent overview of the upgrade process used to move from Hyper-V RTM to Hyper-V R2 (Hyper-V RTM is the final release of the original Hyper-V as opposed to the beta). You can find the Microsoft support document here:
http://support.microsoft.com/kb/957256
The key factors I noticed where that snapshots are not fully compatible between Hyper-V and Hyper-V R2 and saved states are completely incompatible. If you have saved states for virtual machines, you will need to power off those virtual machines before an in-place upgrade. Of course, the knowledge base article also covers that export/import method of upgrading and the backup/restore method.
