brattyc - If you have Nova loaded now, when the KK update hits your device, you will get prompted for which launcher you want to use. You will also have a choice whether you want the selection to be permanent or just that one time.
Whenever a new launcher is installed (or upgraded), you will get prompted for which launcher to use as your default. Also, if you don't like your default selection, you can also change it with: Settings >> Applications, then select your launcher and then "Clear Defaults".
So, you'll always have the option to try whatever launcher you want, and your choice isn't permanent.
AzJazz