Hi,
New versions of Fusion do tend to give the VM new virtual hardware, but the upgrade is small and most small changes are ignored by Microsoft's activation logic.
So far I've not had any problems upgrade virtual hardware triggering the activation logic.
I have seen it happen when changing to another host where the CPU is very different, if the CPU is similar it won't trigger this either.
As we are talking virtual machines here, you can protect yourself by either a backup or a snapshot.
If you are not comfortable doing the full reinstall simply take a snapshot before running the clean re-install.
That's how I generally protect myself against these type of shenanigans.
Of course I also have backups as well.
--
Wil