"let alone, force users to switch to another desktop"
I think you're overestimating how hard is is to switch from Gnome 2 to a suitably configured KDE4. They're not radically different approaches; it's much less of a jump than moving to something as conceptually different to either of them as Gnome 3 is.
As long as you can give people something that works roughly the way they're used to they're not going to worry if it looks a bit different. KDE is flexible enough that you should be able to provide people with something that's similar enough to the old setup that they can find their way around.