If you want more flexibility when upgrading to Windows 8.1, the best way is to get hold of a copy of Windows 8.1 ISO. If you have a valid Windows 8 product key and planning to upgrade to Windows 8.1, you can use the trick outlined in the first post of this thread. Given that the thread currently is 34 pages long, there must be quite some demand for the Win 8.1 ISO. Please remember that when using the setup.exe to download the ISO, you need a computer running 64-bit (32-bit) Windows to download a copy of 64-bit (32-bit) Win 8.1 ISO. Another advantage of having a Win 8.1 ISO is that you can clean install Windows 8.1 with a generic Win 8.1 key and then activate it after installation with your own Win 8 retail key. I just tried it under Bootcamp, and it worked fine. This is very helpful if you need to reinstall Windows 8.1 when you only have a valid Win 8 key. Do a Google search on "Windows 8.1 generic keys", and make sure you use the correct version (Core or Pro) to match your Windows 8 version key. If you have any questions regarding the download, they were probably covered in the thread linked above. One more thing, don't try to use the Win 8.1 generic key to directly download Win 8.1 ISO and it won't work; you'll end up getting an error message with something like "check your Internet connection". You have to start with downloading Win 8 ISO, then cancel it and switch to downloading 8.1 ISO twice as explained in the post. Also this trick doesn't work for OEM key, only for retail (upgrade) key.