I may be wrong here, but becuase of all the changes between 8.5.x and 9.x, the recommended installation is actually to completely remove 8.5.x and then install 9.x.

Not the best answer to be sure, but it is what I have read so far.

Now, if you are using managed mail replicas, synchronized contacts, and roaming, the user should lose very little in the process, but it does complicate any sort of roll-out process for the new 9.x client software.