(sorry for the not really academic English, this is not my native language)
I dissassembled my newly bought Switch 10 to check if the HDD is replaceable. Good news, it is.
First, you have to unscrew all cross-head screws on the back of the keyboard, do not forget to remove the rubber piece to unhide 2 screws.
So, you'll be able to dissassemble the two main part of the keyboard.



Victory ! We can see the HDD !

4 screws attached te HDD to the chassis...
Unscrew these one

And these last two one

Now, the HDD is nearly released, there is only one proprietary cable to remove :

Gently emove the black tape

You can see that the HDD is standard :

Now you have disconnected the HDD from the cable, you can install your new HDD/SSD

My Samsung 850EVO installed

You will notice that thet keyboard is in fact a USB hub, let see how the HDD is recognized :

Hope it will help :-)