Gevonden op het Parallels forum:
I upgraded my Windows XP 32 bit to Windows 7 64 bit using the upgrade disc.
1) Go to configuration for the XP virtual machine and then to Hardware > Boot Order > set CD/DVD to the top position.
2) Insert the Windows 7 64 bit upgrade disc into the CD tray
3) Boot the XP virtual machine but immediately hold Control + F12
4) You get the Windows 7 Install screen.
5) Select Custom install.
6) Select to install into the partition for "System."
Let the computer do the rest.