If you're working with people who are comfortable with technology, then making such a transition should not cause too much pain. Annoyances yes, especially with file format compatibility issues, but nothing too serious. You'll be answering lots of questions, but the questions themselves will be from a position of needing some details filled in, not failure to understand basic concepts.
On the other hand, if you're working with people for whom computers and technology are PFM (Pure @#%$ing Magic) then ANY CHANGE, no matter how trivial, will lead to nervous breakdowns. For such people, use of a computer involves memorized incantations (if not outright prayers) based on mouse movements, clicks, and magic words typed into the screen. If these change, even slightly, they will be utterly lost and terrified -- and they'll blame YOU.
If this is the case, then you're going to have to create a standardized installation of Linux with a normal desktop interface (Cinnamon, KDE) and then TRAIN your employees on how to use it. Mint is a good choice. I'm using the KDE version of Mint 16 on all my workstations. The cinnamon version is also perfectly usable. There are of course other options. The key is to create an environment that is as close to what they know as possible. Not necessarily in terms of how it looks, but how it BEHAVES.
Even so, there will always be some differences that will trip such users up. You guys might have to hire a temp worker whose sole job will be to train and support your employees until they learn the new incantations.
The good news is that moving from XP/Vista/7 to a normal desktop Linux distro will actually be easier than trying to retrain these employees to use the malware that is Windows 8.