If anyone else wonders, the UEFI BIOS for Android is x64, and for Windows x86.
Therefore, if you want to go from Windows <-> Android, you will first have to flash a different BIOS.
But I agree with Villa: I got my hands on a Z64-W that was converted to Android, and since the Z64 only supports KitKat (v4.4.4), I found that most of the apps I normally used were not compatible.
So if anything, I can only imagine someone would ever want to convert an Z64-A Android box to Win10 (or personally I installed Win8) rather than converting a Z64-W to Android and end up stuck with KitKat.
There is also a discussion stating that for Win10, you need a different BIOS from the stock Win8 one; I'm not sure whether it's because the stock BIOS doesn't contain the Win10 activation key? Conversely, I'm not sure if the Win10 BIOS ocntains the Win8 activation key, so to be safe, I kept the old BIOS version.