To get x64 while preserving the Windows 8 key I believe you would first have to upgrade Windows 8 x32 in place to Windows 10 x32, then do a clean reinstall of Windows 10 x64 on the same hardware profile. It should then reactivate automatically.
Yeah, "do a clean reinstall of Windows 10 x64 on the same hardware profile."
That should work on a physical hardware pc but on just a W10x32_VM...that would just be a brand new VM and install W10x64 on it.
Now I have not tried it but can't imagine Microsoft allowing me to have an activated W10x64_VM whenever I wanted to create one.
The"hardware profile" would always be the same as far as I know. Maybe the VM gives different device numbers when a "new" VM is created.
Which is why I wanted to create the W10x64_VM from the old activated one. Hardware pc - no problem, VM...very iffy.