Well, for now at least, they don't. The Z Roadster will continue as a 350Z model, with the legacy styling and the same 3.5-liter V-6 engine under the hood producing the same 306 horsepower. In fact, the only update for 2009 is a new Moonlight White exterior color, which replaces Pikes Peak White. In other words, there's no (real) change whatsoever.
We're sure that it's only a matter of time before Nissan upgrades the Z Roadster with the new drivetrain and styling, there's no way they'll have two radically different Zs on showroom floors for too long. For now, if you want a drop-top Z, you'll have to make due with the 350Z for a while longer.