I am using Windows 8.1 Pro VL MAK. Now it upgraded to Windows 10 Pro and activated. Upgrade process takes several hours as it failed 3 times, and each time, the download start from beginning. I decided to try upgrade using extracted ISO, and it works. After upgrade finished and activated, i check on slmgr -dlv, it still reported under VL MAK, with product key is **-*****. So, I think the default key on the ISO is **-3V66T, which is generic retail key, while for VL MAK is **-*****. Btw, I was thinking to rebuild my computer with Win 7 retail, but since I got notification that the computer eligible for upgrade, I decided to try it. And it works. AFAIK, under Volume Licensing scheme, there are Windows Client Enterprise version or Professional version. And I think, MS fully aware there are a lot of companies are using Enterprise version. Then they decided to blocked the free upgrade for Enterprise, to ensure the money come to pocket. Also, it makes easy to the upgrade scheme, as Enterprise version is only available under volume licensing, while for Pro version, it also available on retail and oem too. Giving free for non-enterprise user is just creating a good marketing for them, while of course, they still need money. Windows & Office are the bread and butter for MS. And I am quite sure it's by design, not a loop hole. Btw, in some cases, big company who using Enterprise version is not directly purchase it. Sometimes, they upgrade it from OEM version to Enterprise version (as part of deal with MS, not technically). The OEM version is acquired as part of the new computer. Anyway, it just my opinion.