While you might be able to trick it into doing the upgrade, you really shouldn't...
Previews are not meant to be used by end customers (you can, but stability isn't guaranteed) and thus will not provide upgrade paths like those for end customers. It doesn't even make sense to introduce an upgrade path from a preview version to the full version. If they were to introduce an upgrade path for this, they would have to deal with a lot of fixes to corruption that was introduced by using the previews, which is one of the main reasons the upgrade path isn't even considered.
If you care for your data and not run into corruption or a buggy OS some time later, install it clean.
Do you use a proxy of some sort? – Michael K – 2012-10-26T08:34:31.673
I'm having this issue as well. I'm in Denmark. I posted about it on Microsoft Community: http://answers.microsoft.com/en-us/windows/forum/windows_8-windows_install/windows-8-isnt-available-for-download/50b43e85-afea-4951-b29e-7ca66164693d.
– Asbjørn – 2012-10-26T09:10:49.647Why did you forget to say you were doing it in windows 8? and why did you run windows 8 upgrade in windows 8? It is already windows 8? – Piotr Kula – 2012-10-26T10:10:26.047
@ppumkin, I did this thing to use the Windows 7 upgrade code for $14.99. You can't enter it anywhere else AFAIK, and I already had Windows 8 RTM setup, just not activated. – joshschreuder – 2012-10-26T12:14:51.850
Check this here to Upgrade from Windows 8 Preview to Full Version: http://superuser.com/a/483380/161601
– FiveO – 2012-10-26T12:17:05.427