The only things I can see taking any time or presenting any unusual problems switching to LINUX would be drivers for new hardware items or needs for unusual software. Should take about four or five weeks, and not five years.
What about all their documents saved with proprietary markup? They'd need to start converting everything and in some cases writing improvements to the open source software to do some of the things they need it to do.
In addition to this there is a lot of software that just doesn't have viable options on linux, things like accounting software (some of the creators of Linux still keep a Windows machine around to do their finances), graphics and video editing, some publishing software....
Linux rocks, but for a large scale company or a government to completely switch to open source platforms is not
as easy as just finding drivers for their hardware.
In the future, as more and more FOSS becomes legitimate competitors to commercial software it will be easier, but there are still some departments of some companies that can't get by on Linux right now.