I initially bought Windows 7 Home Premium because it did all that I wanted. I them did a motherboard/processor/RAM upgrade to 32G memory only to discover that Home Premium didn't support more than 16G. So I bought an upgrade to Pro purely to get the benefit from the extra RAM.
So it looks like a rebuild would entail an initial Windows 7 Home Premium installation followed by an upgrade to Windows 7 Pro to enable a further upgrade to Windows 10 Pro.
No wonder swarfega has invested in a retail version.