Interesting... In Windows 10, this is the scenario:
MKR1000 is plugged in to a USB port on my Windows 10 laptop. I then go in to Arduino (1.6.8 or latest nightly build for today) and I can see the MKR1000 at COM8. OK so far...
Then I load up a bit of code and publish it to the Arduino. It sometimes fails (com port busy, or can't set mask) but when it doesn't fail, it will upload the code to the Arduino which then does the reset CPU... and windows finds it again at COM9. Hmm...
So, I change Arduino IDE to use COM9 so that I can start up the serial monitor. So far it is ok. Now, if I update my script and publish it again, upon reset, Windows finds the Arduino at COM10 !
It then goes back to COM8 if I publish again.
In all this, I have not unplugged the USB cable.
Also, if I press the reset button on the MKR1000, Windows 10 again changes the COM port.
This problem with changing COM ports does not happen with other models of Arduino, just with the MKR1000. I'm in the UK, so it is a Genuino MKR1000 that I have (if it makes any difference).
So... it seems there is a problem with the MKR1000 USB driver for Windows 10.
Anyone else seeing this?