They way xenforo is made makes this very hard and will result in the loss of posts and other data, unless you roll back, use the old database information and copy information over from the newer version. It's not entirely impossible, but not straight foward either
The addon has been removed from the xenforo site now, and i doubt it's ever going to come back.
However, there's an addon that allows you to import all ratings from "Post ratings for xenforo 1" and use it in pretty much nearly the same way, but with the normal rating system. Would there be anything that prevents you from doing this?