It's more usual for open source desktop conversions to switch wholesale... not just Office and Windows but other commercial products as well (from Adobe, etc.)
In most of these conversions the primary reason is political, not financial. In the Munich example, even their original projections acknowledged that it would be cheaper to simply stick with Microsoft products.
So usually the justification for a switch is to be free from vendor lock-in. And that's a big reason why the OO/LO folks do not want OOXML to succeed, because then the vendor lock-in argument can't be used anymore to promote their services.
In smaller cases, the reason for conversion is for security concerns (e.g., Department of Defense, in part to reduce malware exposure). Again it won't make sense to just switch Office to OO/LO in this case, so typically the whole desktop gets replaced.
Aside from some niche industries (e.g., CG/animation), virtually all of the large open source conversion projects we've seen are by governmental or educational institutions. And among smaller companies which have switch away from Windows, most have switched to Macs instead of to Linux.