Everyone who purchased this software should be allowed a free upgrade to a version compatible with the most up to date operating system at no cost to the customer.
I think it would be entirely reasonable to expect that your purchase of a program would remain compatible with whatever OS for which it was purchased. I certainly would expect updates, etc to maintain function/compatibility with each OS Service Pack release, etc.
But with a new Operating System?
No.
Yes, Vista and Windows 7 look similar, and they are definitely 'related', but under the hood they are different, requiring program rewrites, etc. for functional compatibility..