I have been dabbling and playing in Windows 8 since the first Release Candidate was out. I like it. Of course, on my laptop I have installed Classic Shell http://classicshell.sourceforge.net/ which brings back my familiar Start Menu and Desktop. Metro seems ok for tablets, but for a Keyboard and Mouse setup, it drove me nuts.
Anyway, to the meat of the post here… When the RTM was released to MSDN yesterday I downloaded the ISO. I then mounted the ISO natively in my Windows 8 Release Preview install and ran Setup. I tried the upgrade path, the selection was available. After a couple of steps I was presented with the “This is not an available upgrade path, please visit http://noupgradepathforyou for more information”.
I decided to try the Windows 7 work around for this. This means mounting the iso in Read / Write (I just copied the already mounted iso to a new folder) and then modifying /sources/cversion.ini. I changed the values of MinClient and MinServer to 7100. I then ran Setup from that folder, and my upgrade was successful.
Now, keep in mind, that this is not supported, and could end up with a catastrophic failure if some bits are not properly upgraded. But 12 hours in, and things seem stable.