Soprano 2.3.63 – that is the magic version number you need to look out for.
And then once you have updated your kdebase copy to the latest trunk you run your favorite text editor on ~/.kde/share/config/nepomukserverrc. In there you set Soprano Backend=virtuosobackend in the [Basic Settings] section. After that you simply restart Nepomuk as described in the corresponding howto. You can also logout and log back in again but then you won’t be able to provide as nice bug reports.
Once done Nepomuk will convert your database. This can take a loooong time if strigi is enabled. But it will finish. :)
BTW: You need a recent snapshot of Virtuoso 5.0.12 for this to work.