Yesterday i decided to do a complete fresh install of 8.1.. added the ei.cfg so i didnt need to use the install key.. after the first boot windows comes up with a black screen saying now is the time to add a serial .. put my upgrade key in it accepted and activated with no issues.. so yes it appears you can upgrade for nothing and do a clean install
Guys if u all runnin genuine u will all be fine for the upgrade and u guys with OEM.DM bios will be fine for the upgrade too!! Don't worry
I'm going to wait till Thursday and see how it works. If I can clean install via the store I will , If its just an upgrade I will download the official ISO on Friday. The thread you linked is more about Windows 8 Activation issues I had with keys getting mixed up and the ei.cfg file The update is Free to genuine Windows 8 users and will be sold as a stand alone OS as Windows 8.1.
For the win8.1 preview, when people upgraded from Store, it downloads the whole esd file, then proceed to upgrade win8 - so what people did was to "hi-jack" the esd by suspending the process and copy the esd to somewhere else, after that, you can simply replace the wim by the esd in a win8 or 8.1 install ISO. Maybe the process would be the same for the win8.1 GA upgrade.