It will write over what it already installed and install the new stuff.

If you have added non stock content to the runtime it shouldn't mess it up.   I've had to reinstall Poser a few times because of quirky behaviour and my extra things that I added to the Poser 6 runtime have been left untouched.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi