I gave VPC2007 a short spin on WinXP Pro Host.
First preliminary impressions.
If you already have VPC2004 with few Guests that are in Saved Mode, start the Guests and turn them off before you upgrade.
VPC2007 would install on top, and replace VPC2004.
Since it is beta, some screens would say 2003 or 2004 as though it is the old VPC but the end if the About states “Microsoft Virtual PC 601.122.0” you got VPC2007.
You need to install again the Additions. On my system, I needed to go through two boots with each Guest before it “Calmed Down”.
Otherwise, I did not have any loses or any overt trouble.
As on VPC2004; No USB support on VPC2007 Beta.
I tried, Win98se, Win2000, and WinXP Pro as guests, all worked smoothly and slightly faster than before the upgrade.