If you are planning to upgrade to Windows 8 then you should be clear about the structure of the operating system that you want to use i.e. either 32bit or 64 bit.
You can download Windows 8 ISO from Microsoft but in this procedure you will not find any option to choose the structure of the OS.
So what if you had previously upgraded your Windows 7 32bit to Windows 8 32bit but now you are planning to go for Windows 8 64bit version? Don’t worry because you don’t have to purchase any extra license. Your Windows 8 key which was provided to you at the time of first order will still work.
All you need to do is to find some PC which is already running 64bit OS, be it Windows XP or Windows 7. Simply download the Windows 8 Install Software, enter your key for Windows 8 and follow the steps. At the end of the procedure, you will have a geniune Windows 8 64 bit ISO from Microsoft.
For a detailed tutorial about how to download Windows 8 ISO from Microsoft, follow this link.