Quote - Yeah don't for the sake of your own sanity try copying an older runtime into a newer version of Poser, I did & ended up with a non working Poser until I reinstalled it.

The quick and easy way of doing it is to move the new Poser runtime somewhere else, copy the old Poser one to the (now empty) place, and then copy the new runtime back over the old one.
That way you make sure the new Poser will have all the essential files it needs to work.

It works, at least it did work for P6: I did that when I decided to uninstall P5, and didn't want to have a lonely runtime sitting there in the middle of nowhere.