I have Windows 8.1 Pro 64 bit installed on my iMac (20-inch mid 2007). I have downloaded from Microsoft a free copy of Windows 10 Pro 64 bit. Microsoft did not ask me to do so. I decided since Window 10 is free, I would like to try it. I do not have a Product Key. During installation, I want to format my Windows partition and do a clean install of Windows 10. When finished, I what Windows 10 to be activated.
In response to bmike answer, I will add the following.
My Mac's firmware will not boot the 64 bit Windows DVD releases by Microsoft. I have to burn a special DVD where the EFI booting is removed. The procedure is given here. I also have problems installing the Boot Camp Support Software 4.0.4033. The solution to this problem is posted here. So when Apple posts, basically you have to have a 2012 or newer Mac to run Windows 10, I would not believe this to be true.
Microsoft has always incorporated the ability to use previous version drivers to run Windows. This goes all the way back to the days of OS/2.