I've been running with Awesome as my window manager for a while now, but to chase down an issue I've been having with fullscreen lagginess on a 4k monitor, I tried to switch back to KWin temporarily to see if that is my issue, or not.

But when I try to switch back, I get first a dialog stating that the current window manager is going to be replaced with "the configured one", followed by a dialog stating that the new window manager failed to start, and that "the running window manager has been reverted to the default KDE window manager KWin." Which, is not correct, as Awesome is still running. Further, this changes back the selection in the settings to Awesome, so even logging out or rebooting won't change it over.

Is there a way to manually change the window manager in a config file somewhere?

For a tryout, you can just run "kwin --replace &" at runtime.

If this fails, it might be because
a) kwin isn't installed
b) kwin installation is broken (missing/incompatible libraries)
c) kwin has trouble to talk to the X11 server
d) (most likely) kwin fails to take over the WM selection, what means that awesome isn't giving it up on being called for it.

In case of (d) try to kill awesome first: "pkill awesome; sleep 1; kwin &"