No Operating System is Replaceable
So many people out there just love to talk about how Linux is now a "drop in replacement" for Microsoft Windows or Macintosh OSX. This isn't the case. The sad truth is that there are no "drop in replacements" for most software, and especially so for operating systems.
Let's look at Windows for a moment. There are two things that hold people to Windows like super glue. The first one is MS Office. We need to be realistic. OpenOffice.org is fantastic. It's my personal favorite as an office suite. It is NOT 100% compatible with MS Office, and that's just how it is.
Quite often, rulers are going to be off. Advanced formatting will always be skewed, and fonts are never accurate. Telling someone that they need to alter (potentially) thousands of documents to switch to a "free" software stack is effectively making the software stack anything but free.