Upgrade Windows 8.1 to Windows 10
Solution 1:
I updated from Windows 8.1 to Windows 10 on a MacBook Pro 15 inch Retina 2013.
Best is not to do a clean install but choose to upgrade the existing OS, Windows 7 or Windows 8.1. This will avoid a lot of problems with the authentication key you need for Windows 10 and the necessary drivers.
Windows 10 will use the Bootcamp Windows 8. 1 drivers which are already installed on your PC. They all work ok except for the bluetooth driver. It is installed but bluetooth doesn't show up. That's the only problem I had with upgrading to Windows 10.
Solution 2:
My problem was I answered "Don't keep any personal settings and data" when Windows 10 installer asked what to keep and then it deleted all of the Apple drivers including Bootcamp. So my bluetooth, the sound driver and a couple of other devices were not installed and no matter how hard I try I couldn't get them installed. Make sure you would choose "keep everything"
Solution 3:
I made a clean install on my MacBook Air using Bootcamp. I created a bootable USB with Bootcamp. Process was smooth. Downloaded image from Microsoft, showed image to Bootcamp and showed the USB stick to be used.
I installed insider preview but doubt there are any differences. On the partition I installed Windows 10, I had a Windows 7 before; I erased the old partition during installation.
Also, I don't know if it makes difference but I have new OS X beta installed on my Mac.