How important is it for an enterprise to keep up with version changes in desktop operating systems?

Seeking to stretch their budgets in a tough economic environment, IT managers are subjecting every technology purchase to rigorous scrutiny and looking for every opportunity to reduce total cost of ownership (TCO).
Given this mandate, how important is it for an enterprise to keep up with version changes in desktop operating systems? If most employees of a large company are using Windows 2000 or Windows 7, for example, would you recommend upgrading everyone to Window 8.1? Why? Why not? Would you answer differently for a small business as opposed to a large one?

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