I use OS X as my primary operating system, but still need to use Windows so I've installed Windows 8.1 as a VirtualBox Virtual Machine. Recently I decided that I wanted to upgrade to Windows 10, but when I do Windows complains it doesn't like the VirtualBox Graphics Adapter for Windows 8:
I got quite busy, but recently I decided to find out how to resolve this. A (very) cursory Google search wasn't terribly helpful, so I decided to see what registry keys Windows looks for when it checks for compatibility. For this task I used Process Monitor - the process is called GWXUX.exe so I restricted the capture to only the registry and added the filter: Process Name is GWXUX.exe
A quick scroll through the output showed that there is a registry DWORD HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\OSUpgrade\AllowOSUpgrade
This is set to 0x0, but if you change it to 0x1, then you can upgrade in Windows Update itself (note, if you click on the upgrade to Windows 10 icon in the system notification tray it will still run the compatibility check).
Hopefully someone will find this useful. I can only imagine VMware installs might be getting the same issue, so this key will force an upgrade. I should note that if you set this key on anything other than VirtualBox and do an OS upgrade you do so at your own risk :-)