The only thing that is not saveable yet to an upgrade, is the Control Center or Desktop (look and feel) configuration.
I hope the Desktop configuration can be saved in new VL releases.
KDE settings for users are stored in the /home/username/.kde and /home/username/.config directories. In order for the new updated menu, icons general configuration files of the newer isntalled VL version to present itself to users after installation the skeleton directories (e.g. .kde and .config) are copied automatically from /etc/skel/ when a new user is created. To not do this means the user would be totally confused with an old menu from an old system - an unworkable situation.
So I don't really think what you suggest would be a feasable option.
Furthermore, it would be nice to be able to configure a Desktop and have the option to apply this configuration to any other user, instantly.
You can do this by logging in as the new user and copying across the newly configured .kde and .config directories from the other user, in about 3 seconds:
cp -r /home/olduser/.kde /home/newuser/.kde
cp -r /home/olduser/.config /home/newuser/.config