I asked for confirmation to be doubly sure. The spokesperson said:
"Computerworld got it right. Assuming the customer had a previous version of Windows installed before Release Preview, they’ll be able to upgrade from the Release Preview. They won’t need to reinstall the previous version to do the upgrade; they can just upgrade on top of the Release Preview." Computerworld also noted that when and if Windows 8 Release Preview testers upgrade, they won't be able to preserve their system settings or applications; only personal files will be moved automatically to Windows 8. This is more like a "migration" than an update, and sounds a lot like what will happen to Windows XP users who opt to move to Windows 8.
Again, Microsoft has confirmed this information, as well. After posting details about the deal on July 2, Windows Communication Manager Brandon Leblanc added an update about the ability of Windows 8 testers to upgrade. He said (in the comments of the post: "(P)eople can move from the Windows 8 Release Preview to Windows 8 Pro using the Windows 8 Upgrade Assistant and will only have the options to migrate their personal files or keep nothing at all when upgrading. People moving from the Windows 8 Release Preview will need to make sure you have an underlying license for either Windows XP, Windows Vista or Windows 7."
One last thing to remember: This $40 upgrade price is currently positioned as a promotional price. The offer will be good from whenever Windows 8 is generally available until January 31, 2013. Microsoft may or may not extend the end date.