Dell XPS 13 (2015) Developer Edition - upgrade from linux to Win10

Thank you. Does that upgrade void the Dell warranty?

No upgrade to Windows 10 for free for you.

If you want Windows 10, you will need to buy a license.

Thank you for responding, but the term "may" in your response is precisely the reason for my question: Is it a Dell policy to (a) deny assistance, on the basis that the system was purchased with Linux and is now running Win10, but at the same time (b) provide assistance to a system that was purchased with Win7/8 and is now running Win10? It appears inconsistent, but I need to confirm it either way.

No, however if you need assistance they may have you re-install the factory supplied OS to the computer before providing you help unfortunately.

From my experience and from what I've read on these forums, Dell will only provide you in warranty assistance with stock hardware/software issues.  While you are allowed to put any OS you wish on your computer, you didn't pay for a 'license' from Dell for windows so they will not help you.  If you purchase windows 10, all support in regards to the OS itself will have to go through Microsoft depending on their policies and procedures.  

For users upgrading to Windows 10 from another Windows OS via the 'free' upgrade, that is a mixed bag as Dell has been providing drivers for supported machines, but pushing people to talk with Microsoft for any OS related problems if the system did not come with Windows 10 to begin with.

If Dell list Windows 10 drivers for the model then you may upgrade.

They are still going to support the hardware (which is the important part).