Not really, and no. This shouldn’t affect your already-running system. This change means that the iso will offer plasma by default and will run plasma in the live environment.

And I wouldn’t say it’s particularly hard to switch from any desktop environment to another. It takes some relearning where stuff is, keyboard shortcuts, etc, but any desktop environment can run any Linux program, provided the necessary libraries are installed (which your package manager takes care of). You can install kde programs on your xfce desktop, and they will run fine (and vice versa). They’ll just pull in a bunch of kde libraries when you install.