Having asked questions on this forum and the W7 forum, I have successfully installed W10hp 32bit on an old socket 939 Athlon X2 4800+ machine. It was suggested that 64 bit was also likely to work. Microsoft's upgrade would not work because the processor lacked the correct instruction set. I found that a clean install from an iso, I burned a dvd, but a usb would also have worked. The 32bit installation only used 11.5gb of the ssd and activated fine with the W7 product key.
I wonder how many people who want 10 have been put off by the upgrade path requiring higher specs than a clean install?