I just wrote this up for a coworker who asked me how to move a virtual machine from being hosted by Virtual Server to Hyper-V. First off, I'd like to say that I am a big fan of this type of upgrade. The first step to improving your virtualization experience is realizing that your experience could be improved, after all :)
In any case, it should be a fairly trivial move and would function on Virtual PC 2004/2007 machines, as well. The only thing you'll need is the vhd(s) of your current virtual machine.
That should basically be it all you need -- everything else should just work. Note that you won’t be able to use external networking with a normal Hyper-V network adaptor until you have the Integration Services installed. If you need network before that you’ll have to shut the virtual machine off and add a Legacy Network Adaptor. For example, you would need this if you have Win2003 prior to SP1 since the Integration Services require SP2.
Enjoy, and happy virtualizing!