This may be an option for some - we recently did it in work...
1. Take an image of your current OS Cortex machine (we used Acronis True Image)
2. Go ahead and do the free upgrade to Windows 10
3. Take an image of the Win10 setup
4. Roll back to your previous image until Cortex update available
There have been no licensing issues we've seen with this so far but check it out before you try it.
M.