Google's MVNO project relies heavily on the same technology that makes Google Voice work, but you're not losing anything by using Fi.
News that Google was finally pushing Project Fi invites to a handful of people led to a whole lot of users asking those lucky enough to get in on the action what the experience was like. In doing so, a screenshot appeared that suggested using Project Fi meant destroying your Google Voice account. The dialogue box from the screenshot explains you can either bring your Google Voice number over to Project Fi, or you can use a different number with a caveat explaining your current Voice number will be released if you go with a different number.
If all you know about Project Fi and Google Voice comes from that one screenshot, there'd be plenty of reason to be concerned. Fortunately, after a quick chat with the folks at Google, we know things aren't nearly as dire as they seem.
Project Fi uses the same infrastructure as Google Voice, and all of those features follow with you from Google Voice to Project Fi.
The first thing you need to know about Project Fi is how the system is built to function. Fi operates on a special hybrid network that allows users to move seamlessly between Wifi, T-Mobile, and Sprint networks, with unique pricing that lets you pay for what you use and not a penny more. The service also lets you use your Fi number on devices that aren't connected to the Fi network. You can use your Fi number to make calls, send SMS, and access call forwarding and voicemail transcripts on anything running Google Hangouts, including your desktop.
Sound familiar? That's because Project Fi uses the same infrastructure as Google Voice, and all of those features follow with you from Google Voice to Project Fi.
Google Voice isn't being destroyed by this new service, it's growing up and becoming Project Fi. You have the same features that work the same way, with the added bonus of being able to use the Project Fi mobile carrier service if you choose. Remember, Project Fi is month to month, so you can jump on and off the service as you please. More importantly, Google has confirmed users who stop using Fi will be able to continue using the Google Voice features in Hangouts, only now it'll be called Project Fi when setting it up on new devices. If you want access to your old Google Voice information, like call history and old voicemail, there's a button to do exactly that inside the Project Fi web interface.
So relax, Google isn't taking Voice from you. If anything, Google is doing what folks have been begging them to do for well over a year now by bringing Voice into the modern era and tacking new features onto it. If you're only casually interested in Fi and don't want to move your Voice account over, you can use another Gmail account and get a fresh number generated. Otherwise, sit back and enjoy the ride.