Well, the thing is... if you still had 9.x installed, I wouldn't recommend it, because we haven't tested this direct upgrade path where the installer takes over all config files for the web applications. As you're basically doing a fresh install (except for user directory and message store), that's not a big problem. Just try to make sure you set "most" of the parameters during install to be the same as before and you should be good. If not, we'll iron out the issues.
One more thing - if you still have your 9.x server running, you might want to run a
command while all your users are logged out. This would ensure that your message store is consistent before the upgrade.
Cheers,
Florian.