The previews once expired will automatically restart hourly, making messages to pop-up letting users know of the need to upgrade.
The company had earlier warned on the fate of using the Windows 8 previews; stating that "from the expiration date, you may not be able to access unsaved data used with the software. Any applications you receive from the Windows Store will also cease to be available to you in the future versions, unless they are made available for re-download and you re-acquire them. You may not receive any other notice".
Microsoft support document, however, noted that to continue using Windows Developer preview or Windows Server Developer preview, you must install the update before the expiration date.
The discounted upgrade to Windows 8 pro is fixed at $39.99, which last through to January 31, 2013. Then the cost could go as high as $199.99.