Was looking to upgrade my bootcamp install from Windows 7 to Windows 10, but I didn't see anything on the web about handling this. My guess is that there were a few folks too scared to run an upgrade and instead decided on a clean install. 

Upgrading Windows 7 in bootcamp, to Windows 10 is incredibly painless and took less than an hour once I had all of my files backed up. 

Couple of things needed to get started. 

• Flash Drive, I used a 16GB, can get away with an 8GB if that is what is available. 
• Microsoft USB/DVD download tool. Available here. 
• Image of Windows 10 release. ISO format is what you want. 
• Large External backup of some kind. Your data should always be backed up before an operating system upgrade. 

All next steps are to be done at your own risk!

I'm not going to include screenshots in this as the setup is incredibly simple. It's a good idea to get all of your tools in one spot, I loaded everything I needed to get started on my Windows 7 desktop just for ease of access. 

• Restart your Mac into Windows. Just as a reminder, restart your Mac> once you hear the chime hold down the 'alt/option' key>Select Windows. 

• I like to use a tool called BootChamp created by Kevin Wojniak @ Kainjow.com to help speed up this process. It's a great tool for switching from sOSX to Windows on the fly. 

• Install (if not installed already) the Microsoft USB/DVD download tool. 

• Launch the Microsoft USB/DVD download tool and navigate on your machine to the ISO that you will use to upgrade. Follow the prompts to begin the process. This will format & erase the data on the flash drive to create the installer. 

• Once it's complete use Explorer to open the flash drive contents. Start by clicking Setup.exe

• From here the Windows 10 installing will start inspecting your system to ensure that you meet the minimum requirements. To get the full effect of just how slick this install process is I recommend allowing it to an upgrade to your existing OS. Allow the installer to check for updates. Better now than later. 
• If there are any incompatible programs the Windows installer will give you a heads up and ask to remove.

• Now that all of your settings are ready to go, the Windows Installer will begin copying files and reboot your system. Over the course of the install you'll have a total of 4 reboots before you're completely ready to enjoy the OS.

• As with a manual reboot into Windows on your Mac, once the system restarts and you hear the chime hold down the 'alt/option' key and select Windows, not the install flash drive, but your bootcamp partition. This will begin the 'copying' files stage of the process. When done the system will reboot. 

• If the screen goes black during the install that's the Windows installer being nice to your display. You can shake your mouse and see progress again!

• Hear the chime and hold down the 'alt/option' key again. Select Windows. This will now begin installing files/devices part of the setup. Watch the install and wait again for the reboot. 

• Hear the chime and hold down the 'alt/option' key again. Select Windows. Now it's finishing the install configuration! Almost done. Wait for the reboot!

• From here Windows will take you through a few questions now that the setup is almost complete. You will have to watch the "This won't take long" & "Setting up your apps" messages for a moment, but once they're complete you're ready to go. 

• Now that the new OS is installed and ready for use Windows 10 will give you a quick tour of some new features and ask that you setup Cortana along with your Microsoft account. 

Hope anyone else who was looking to do exactly this found the guide helpful. Feel free to send a message with any questions or feedback. 

-Gerald.