haven't tried this out. though, linux is said to be capable to "change files on the run", so that even running apps can be updated while they're in use..so basically I'd understand it so that you should be able to do it. I haven't heard of anybody who had said it won't work...
I'd say that if you definitely want everything to run ok, then the only way to be sure (if it's even possible) is to first shut down KDE. but on the other hand, why not?
I'm quite sure it works.
there's still one thing - I read this while upgrading my graphics card's drivers. when updating this kind of stuff, you shouldn't use the app it's affecting...in my driver's case, it said I should shut X down until the driver install & configuration was ready, to avoid problems. that is, because the drivers work somehow on a "lower level" or something...don't know how to put it
like they'd be more vital or something. argh, can't get it said right...
but anyway, I think you don't have anything big to be afraid of (unless someone has better information)....KDE is still a bunch of apps running on your X, and if updating apps works (you can update your portage through portage
hehe), KDE should be no exception.