From Microsoft when asked what happens when the final full version is released.
I quote.
"You will either need to purchase the final release or reinstall the version of Windows that was originally installed on your machine prior to installing the Release Preview.
Q: Will I be able to upgrade from pre-release versions of Windows 8 to the final release?

For those of you who have already been running the Windows 8 Developer Preview or Consumer Preview, you can install the Windows 8 Release Preview using the migrate option (just keep personal files), but not the upgrade option (keep personal files, apps, and settings). Or if you prefer, you can of course do a clean installation (keep nothing). The Consumer Preview release does permit upgrading from Windows 7, and will run the integrated upgrade advisor to check on any things you might need to look into. Please keep in mind that there is no rollback after an upgrade installation. We also strongly recommend that you perform a system backup prior to an upgrade, migrate, or clean install of Windows 8 Consumer Preview.
Also note that the final release of Windows 8 will not support upgrading from any prior Windows 8 "Preview" release, though the migrate option will still be supported. In any upgrade scenario, you can run the Disk Cleanup Wizard to remove the previous installation in order to free up disk space. The download will also support boot from USB for a completely clean installation as well."

So be aware and DO NOT install over your existing Windows or if you do be prepared to either pay up an unknown amount or do a clean installation of you old version. The offer is just another way for MS to suck out as much money from users they can.
They don't even offer a rebate of what you pay for this per-release.

Personally they can forget the offer.