You should be able to install Fusion 8, then just upgrade and boot your existing VM. If it triggers revalidation, you can call MSFT (they'll give you the number on the screen) to revalidate the OS install.
This sounds a lot easier, and I think it is a good idea to give it a try because it is so much easier.
--> I would make a complete backup of the Windows virtual machine file first.
If something goes wrong and you call Microsoft, and they refuse to activate your Windows virtual machine for some inexplicable reason, then you can restore a copy of your virtual machine from the backup and try the long convoluted approach instead. Be sure you keep a safe, intact backup until you are sure everything is working. For that matter, you should make backups of your virtual machine anyway. Time Machine does not do it correctly.
Hint: You can right-click your virtual machine file and choose Compress from the contextual menu in the Finder. It will compress your virtual machine to a ZIP file and use only half the disk space as a copy. As wila recommended, it is a good idea to keep your backup on an external drive if you have one.