[CentOS] CentOS 7 and display managers

<rant>
Dear gnome developers - could you *possibly* be more anti-Unix? I mean,
thanks *so* much for trying to turn Linux into Windows or Macs....
</rant>

So, now that I've gotten that out, the KDE display manager, on the login
screen, easily lets you choose window managers. Gnome utterly refuses to
consider such an idea.

I've just yum groupinstall "KDE Plasma Workspaces" on one of my user's new
system... and I cannot figure out, not in googling, and there's nothing
vaguely obvious anywhere, how to change to KDE from gnome.

Anyone got a pointer?

        mark, frustrated